Centripetal Vs. Centrifugal
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See if you can sort this one out 'cos it beats me.
A man on a bus has a helium filled baloon on a string. When the bus turns the man predictably moves towards the outside of the corner. The balloon however moves to the inside of the corner. centrifugal or centripetal? Buoyancy or what? I have no idea.
It does work. I tried it not long ago.
A man on a bus has a helium filled baloon on a string. When the bus turns the man predictably moves towards the outside of the corner. The balloon however moves to the inside of the corner. centrifugal or centripetal? Buoyancy or what? I have no idea.
It does work. I tried it not long ago.
Lu, I think it's correct to say that the centripetal forces hold the subject in his seat, and as the centrifuge starts to move, his bodily fluids are accelerated along with the rest of him by forces from the walls of arteries, etc.
As the centrifuge whirls, the things that are fixed to it, such as the subject's body, are continually forced to move in a circle by centripetal forces.
The fluids are not directly 'connected', though, so at one point in time, they will be moving in a particular direction (in a straight line tangential to the circle). The subject's body, however, is moved off that tangential path, and so the fluids will tend to continue along their straight path as far as they can, i.e. flowing along paths of least resistance towards the outside of the circular path.
Then, when all the available spaces have been filled, so to speak, you will end up with the person going round in a circle and his bodily fluids bunched up towards the local gravity (the outside of the circle).
So really, it's not a force that's making the fluids go to the outside, it's just their natural tendency (as with any particular substance) to keep going in a straight line until acted on by another force (in this case, centripetal force provided by the push towards the middle of the circle provided by the subject's outer 'walls'.
My two bob's worth!
Gaseous,
I think what you're talking about is just that less dense things will float on top of dense things (with 'downwards' being the direction of local gravity).
The bus goes around the corner. The air, not being directly connected to the bus, tries to keep going in a straight line, until it hits the walls and has to bunch up.
Now because there is more air towards the outside of the turn (not a lot, mind you, but enough to have an effect), the helium balloon (being less dense) gets crowded out of that side and deflects to the inside of the turn.
You can make a simple accellerometer using this principle - put a bit of styrofoam in a jar full of water and put the lid on. Whichever way the jar accellerates, the foam moves that way too.
[ 28 December 2001: Message edited by: Arm out the window ]</p>
As the centrifuge whirls, the things that are fixed to it, such as the subject's body, are continually forced to move in a circle by centripetal forces.
The fluids are not directly 'connected', though, so at one point in time, they will be moving in a particular direction (in a straight line tangential to the circle). The subject's body, however, is moved off that tangential path, and so the fluids will tend to continue along their straight path as far as they can, i.e. flowing along paths of least resistance towards the outside of the circular path.
Then, when all the available spaces have been filled, so to speak, you will end up with the person going round in a circle and his bodily fluids bunched up towards the local gravity (the outside of the circle).
So really, it's not a force that's making the fluids go to the outside, it's just their natural tendency (as with any particular substance) to keep going in a straight line until acted on by another force (in this case, centripetal force provided by the push towards the middle of the circle provided by the subject's outer 'walls'.
My two bob's worth!
Gaseous,
I think what you're talking about is just that less dense things will float on top of dense things (with 'downwards' being the direction of local gravity).
The bus goes around the corner. The air, not being directly connected to the bus, tries to keep going in a straight line, until it hits the walls and has to bunch up.
Now because there is more air towards the outside of the turn (not a lot, mind you, but enough to have an effect), the helium balloon (being less dense) gets crowded out of that side and deflects to the inside of the turn.
You can make a simple accellerometer using this principle - put a bit of styrofoam in a jar full of water and put the lid on. Whichever way the jar accellerates, the foam moves that way too.
[ 28 December 2001: Message edited by: Arm out the window ]</p>
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Arm out the Window said it all really, fluids will tend to follow their straight line, the body cannot and is held in place by centripetal force, same principle as the merry go round with a floor that drops away, centripetal force holds you from dropping down.
The helium baloon experiment works well in a car. tie the balloon to the back seat, when the car accelerates, the balloon moves forward as it moves toward areas of less pressure.
Lu, you are getting worked up over nothing, an "apparent force" does not make it a force. Just accept the fact that some terms are used in place of the correct scientific or engineering term. If a rotor blade breaks off in flight, it will travel in its straight line tangental to its circular path, its just that centripetal force stopping working and thats why it let go. No big deal, no big secret, just terminology.
The helium baloon experiment works well in a car. tie the balloon to the back seat, when the car accelerates, the balloon moves forward as it moves toward areas of less pressure.
Lu, you are getting worked up over nothing, an "apparent force" does not make it a force. Just accept the fact that some terms are used in place of the correct scientific or engineering term. If a rotor blade breaks off in flight, it will travel in its straight line tangental to its circular path, its just that centripetal force stopping working and thats why it let go. No big deal, no big secret, just terminology.
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sling load,
The old balloon demonstration is a great way to help understand the concept of real vs apparant forces.
There is no significant pressure gradient in the air in the car, the balloon simply responds to the "bouyant force" which is an apparant force like "centrifugal Force". If the car were half full of water, you would see a slope as the water responded to the new "gravity field" that is the sum of the centripital acceleration and the gravitational acceleration. If we floated a toy moored mine in the water, we would watch the mine float the way the balloon does.
A bubble level does the same thing, of course.
The old balloon demonstration is a great way to help understand the concept of real vs apparant forces.
There is no significant pressure gradient in the air in the car, the balloon simply responds to the "bouyant force" which is an apparant force like "centrifugal Force". If the car were half full of water, you would see a slope as the water responded to the new "gravity field" that is the sum of the centripital acceleration and the gravitational acceleration. If we floated a toy moored mine in the water, we would watch the mine float the way the balloon does.
A bubble level does the same thing, of course.
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After the introduction (by more recent posters) of the “artificial gravity” and “buoyancy” of the centrifuge and the discussion of the direction of the flow of liquids (such as a pilot’s blood) in this environment, I’d like to try a more complete discussion of the centripetal/centrifugal system. I hope this is not too long winded.
The thing that struck me (I hadn’t thought of it earlier) was the display of “gravitational” properties within the centripetal/centrifugal system. What struck me was why denser material (such as in the lab centrifuge) settles towards the “outside” of the arc away from the center of rotation, and why lighter material (such as the helium balloon in the car) settles toward the “center” of the arc. That sounds right to us instinctively, but in a “real” gravity environment the denser material settles toward the “center” of the gravitational field, and not towards the outside of it. I really had to think about this for a day or more to understand why centripetal force created “gravity” seems to work in a manner that is opposite from “real” gravity. The answer turned out to be very interesting.
First I discovered I had to agree with heedm, that the rotation of an object around an arc is in fact being subjected to an “acceleration” force, which is the centripetal force. But I accepted this fact with certain qualifications, which I’ll explain. The following quote is from the online version of Microsoft’s Encarta Encyclopedia, from an article entitled “Acceleration (velocity)”.
“Acceleration (velocity), also known as linear acceleration, rate at which the velocity of an object changes per unit of time. Acceleration is a vector—that is, it has both magnitude and direction. Acceleration is uniform if the rate of change of an object's velocity is the same over successive and equal time intervals. For example, an object that is released and allowed to fall freely towards the ground is accelerated uniformly. An object tied to a string and swung at a constant speed in a circle above a person's head is also accelerated uniformly; in this case, the acceleration vector points along the string toward the person's hand.”
The quote is confusing in that it basically states that changes in velocity (distance covered per unit of time) is a fundamental characteristic of “acceleration”. Then the article states that tying an object to a string and swinging it over your head at a constant speed is also imparting uniform acceleration to that object. My question regarding the over-the-head description was, “what happened to the “change in velocity” characteristic of acceleration?
Then it hit me, there must be in fact 2 different types of acceleration, “velocity” acceleration, and “directional” acceleration. Pure “velocity” acceleration is applied at 0 degrees (or 180 degrees) relative to an object’s current line of travel, so that only its velocity is changed but its direction remains unchanged. Pure “directional” acceleration is applied at 90 degrees (or 270 degrees) relative to an object’s current line of travel, so that only its direction is changed, but its velocity remains unchanged. Any application of “acceleration” between 0 and 90 degrees applies a combination of both types.
Pure “directional” acceleration, applied at right angles to the line of travel, does NOT alter the velocity of a moving object, only its direction. Pure “directional” acceleration also has to maintain this 90-degree angle of applied force, even while the object’s direction of travel is in the process of being changed. A mechanical system of rotation, with a mechanical means of applying a centripetal force around a constant radius, satisfies this requirement perfectly, and produces pure “directional” acceleration. This acceleration however, has to overcome the inertia of the object being accelerated “directionally”, just as it does when an object is accelerated with pure “velocity” acceleration.
Why physicists don’t describe “acceleration” as existing in both types I don’t know. In a field like ballistics for example, both types of acceleration ARE dealt with, and both are often dealt with separately. In ballistics, longitudinal acceleration and vertical acceleration (from the force of gravity) are at right angles to each other relative to the projectile, just as “velocity” and “directional” acceleration are at right angles to a moving object’s line of travel.
Now that “centripetal acceleration” has been dealt with, let’s move on to the relationship of mass, inertia, gravity and how gravitational properties are displayed in a centripetal/centrifugal system.
The reason that centripetal/centrifugal systems demonstrate gravitational properties is hinted at in the following quote from another Encarta article entitled “Mechanics”.
“A massive object will require a greater force for a given acceleration than a small, light object. What is remarkable is that mass, which is a measure of the inertia of an object (inertia is its reluctance to change velocity), is also a measure of the gravitational attraction that the object exerts on other objects. It is surprising and profound that the inertial property and the gravitational property are determined by the same thing. The implication of this phenomenon is that it is impossible to distinguish at a point whether the point is in a gravitational field or in an accelerated frame of reference. Einstein made this one of the cornerstones of his general theory of relativity, which is the currently accepted theory of gravitation.”
The quote from the “Mechanics” article points out the remarkable direct relationship between an object’s mass (or inertia) and its gravitational attraction. A lighter object has low inertia, but also have low gravitational attraction to other objects. A heavy object has higher inertia, and a proportionately higher gravitational attraction to other objects. I’d like to use 2 simple examples to illustrate the direct relationship between an object’s inertia and its gravitational attraction. The first example will illustrate the inertia of 2 objects, and the second example will illustrate the gravitational attraction of the same 2 objects.
The first example is 2 steel balls on a flat low friction surface. One ball weighs 1kg and the other weighs 10kg. Both balls are at rest (relative to the flat surface). Now lets apply a 1 Newton force (as pure “velocity” acceleration) to each ball for 1 second. Recall the basic formula for acceleration which is: a=F/m (F=Newtons, m=mass, a=acceleration). Also lets use the following formula to help calculate the resulting acceleration: v2=v1+at (v2=final vel, v1=init vel, a= accel, t=time). Recall that 1 Newton is equal to 1kg/m/s.
For the 1kg ball, the final velocity is v2 = v1 (0) + a (F (1kg/m/s)/ m(1kg)) * t (1 sec), which is 1 meter/sec.
For the 10kg ball, the final velocity is v2 = v1 (0) + a (F (1kg/m/s)/ m(10kg) * 1 (1 sec), which is .1 meters/sec.
The inertia of the 10kg steel ball offered 10 times more resistance to the “acceleration” force than the 1kg steel ball, resulting in 1/10th the final velocity of the 1kg ball. You can call the resistance to acceleration a “reaction force” if you’d like, but that would just be semantics, as inertia is a very real force. Again, this inertia is just as real in “directional” acceleration as it is in “velocity” acceleration.
Now let’s use the same steel balls for the second example. This time lets drop them from a height of 10 meters. Discounting air resistance, we know that if you release the steel balls at the same time, the steel balls will hit the ground at the same time. We also know that if you remove the air from a glass cylinder and do the same experiment with a feather and a steel ball, they will also both hit the ground at the same time.
Now here’s the remarkable thing about this second example. We know that gravity is an acceleration force, and has an acceleration value of 9.8 m/s. We also know from the first example that the 10kg ball has 10 times the inertia (resistance to acceleration) that the 1kg ball has. The difference in inertia would be even greater between the feather and the steel ball. So why do all the objects hit the ground at the same time, given that the same acceleration is being applied to all the objects, all of which have different masses and inertia? The answer is that the amount of gravitational attraction exerted by each object towards the earth varies directly with its mass. The greater the mass, the greater the gravitational attraction. This means each object will experience the same units of acceleration (velocity change), because the amount of gravitational attraction between each object and the earth will vary based on each object’s mass. So the more inertia an object has, the greater the attraction to overcome that inertia. The differences in attraction MUST be directly proportional to the differences in mass (or inertia) or else the objects could NOT hit the ground at the same time.
In a normal gravity environment where buoyancy is concerned, the denser objects (density expressed as kg/cm^2) will go to the bottom, due to the greater gravitational attraction of the denser objects. In an artificial gravity environment created by a centripetal force, there is no gravitational attraction pulling any of the objects towards the center, since the acceleration around the arc is mechanical and not gravitational. Thus the mass of the object(s) cannot use their innate gravitational attraction to help draw them towards the center of the arc. Instead, the inertia of the objects resists being drawn into travel around an arc, because the “directional” acceleration is turning them away from straight-line travel. The greater the mass, the greater the inertia and the greater the resistance to being drawn into the arc by the “directional” acceleration of the centripetal force. So in this environment, the greater inertia of the denser objects (or material) causes them to settle towards the outside of the arc. That’s how the lab centrifuge works.
BTW, the “centrifugal” force is real, and is nothing more than the inertia of an object resisting the purely “directional” acceleration of the “centripetal” force. Again remember the formula for the centripetal force, which is:
F = m * v^2 / R (F = centripetal force, m = mass, v = velocity, R = radius)
Note the things in the formula that increase the value of the centripetal force. Increasing the mass will increase the force, because of the greater inertia of the greater mass. Increasing the velocity will increase the force squared, since increasing the velocity increases the rate of the “directional” acceleration of the same mass around the arc (more degrees of arc are covered per second). The greater the change rate in direction, the greater the resistance to that change, and this term is squared. To me, nothing more clearly illustrates the effect of inertia on a centripetal/centrifugal system than a velocity change. Decreasing the radius also increases the force, as this also increases the rate of the “directional” acceleration (again, more degrees of arc are covered per second).
P.S. I wish it was easier to write formulas in this forum.
(edited for typos and greater clarity)
[ 31 December 2001: Message edited by: Flight Safety ]</p>
The thing that struck me (I hadn’t thought of it earlier) was the display of “gravitational” properties within the centripetal/centrifugal system. What struck me was why denser material (such as in the lab centrifuge) settles towards the “outside” of the arc away from the center of rotation, and why lighter material (such as the helium balloon in the car) settles toward the “center” of the arc. That sounds right to us instinctively, but in a “real” gravity environment the denser material settles toward the “center” of the gravitational field, and not towards the outside of it. I really had to think about this for a day or more to understand why centripetal force created “gravity” seems to work in a manner that is opposite from “real” gravity. The answer turned out to be very interesting.
First I discovered I had to agree with heedm, that the rotation of an object around an arc is in fact being subjected to an “acceleration” force, which is the centripetal force. But I accepted this fact with certain qualifications, which I’ll explain. The following quote is from the online version of Microsoft’s Encarta Encyclopedia, from an article entitled “Acceleration (velocity)”.
“Acceleration (velocity), also known as linear acceleration, rate at which the velocity of an object changes per unit of time. Acceleration is a vector—that is, it has both magnitude and direction. Acceleration is uniform if the rate of change of an object's velocity is the same over successive and equal time intervals. For example, an object that is released and allowed to fall freely towards the ground is accelerated uniformly. An object tied to a string and swung at a constant speed in a circle above a person's head is also accelerated uniformly; in this case, the acceleration vector points along the string toward the person's hand.”
The quote is confusing in that it basically states that changes in velocity (distance covered per unit of time) is a fundamental characteristic of “acceleration”. Then the article states that tying an object to a string and swinging it over your head at a constant speed is also imparting uniform acceleration to that object. My question regarding the over-the-head description was, “what happened to the “change in velocity” characteristic of acceleration?
Then it hit me, there must be in fact 2 different types of acceleration, “velocity” acceleration, and “directional” acceleration. Pure “velocity” acceleration is applied at 0 degrees (or 180 degrees) relative to an object’s current line of travel, so that only its velocity is changed but its direction remains unchanged. Pure “directional” acceleration is applied at 90 degrees (or 270 degrees) relative to an object’s current line of travel, so that only its direction is changed, but its velocity remains unchanged. Any application of “acceleration” between 0 and 90 degrees applies a combination of both types.
Pure “directional” acceleration, applied at right angles to the line of travel, does NOT alter the velocity of a moving object, only its direction. Pure “directional” acceleration also has to maintain this 90-degree angle of applied force, even while the object’s direction of travel is in the process of being changed. A mechanical system of rotation, with a mechanical means of applying a centripetal force around a constant radius, satisfies this requirement perfectly, and produces pure “directional” acceleration. This acceleration however, has to overcome the inertia of the object being accelerated “directionally”, just as it does when an object is accelerated with pure “velocity” acceleration.
Why physicists don’t describe “acceleration” as existing in both types I don’t know. In a field like ballistics for example, both types of acceleration ARE dealt with, and both are often dealt with separately. In ballistics, longitudinal acceleration and vertical acceleration (from the force of gravity) are at right angles to each other relative to the projectile, just as “velocity” and “directional” acceleration are at right angles to a moving object’s line of travel.
Now that “centripetal acceleration” has been dealt with, let’s move on to the relationship of mass, inertia, gravity and how gravitational properties are displayed in a centripetal/centrifugal system.
The reason that centripetal/centrifugal systems demonstrate gravitational properties is hinted at in the following quote from another Encarta article entitled “Mechanics”.
“A massive object will require a greater force for a given acceleration than a small, light object. What is remarkable is that mass, which is a measure of the inertia of an object (inertia is its reluctance to change velocity), is also a measure of the gravitational attraction that the object exerts on other objects. It is surprising and profound that the inertial property and the gravitational property are determined by the same thing. The implication of this phenomenon is that it is impossible to distinguish at a point whether the point is in a gravitational field or in an accelerated frame of reference. Einstein made this one of the cornerstones of his general theory of relativity, which is the currently accepted theory of gravitation.”
The quote from the “Mechanics” article points out the remarkable direct relationship between an object’s mass (or inertia) and its gravitational attraction. A lighter object has low inertia, but also have low gravitational attraction to other objects. A heavy object has higher inertia, and a proportionately higher gravitational attraction to other objects. I’d like to use 2 simple examples to illustrate the direct relationship between an object’s inertia and its gravitational attraction. The first example will illustrate the inertia of 2 objects, and the second example will illustrate the gravitational attraction of the same 2 objects.
The first example is 2 steel balls on a flat low friction surface. One ball weighs 1kg and the other weighs 10kg. Both balls are at rest (relative to the flat surface). Now lets apply a 1 Newton force (as pure “velocity” acceleration) to each ball for 1 second. Recall the basic formula for acceleration which is: a=F/m (F=Newtons, m=mass, a=acceleration). Also lets use the following formula to help calculate the resulting acceleration: v2=v1+at (v2=final vel, v1=init vel, a= accel, t=time). Recall that 1 Newton is equal to 1kg/m/s.
For the 1kg ball, the final velocity is v2 = v1 (0) + a (F (1kg/m/s)/ m(1kg)) * t (1 sec), which is 1 meter/sec.
For the 10kg ball, the final velocity is v2 = v1 (0) + a (F (1kg/m/s)/ m(10kg) * 1 (1 sec), which is .1 meters/sec.
The inertia of the 10kg steel ball offered 10 times more resistance to the “acceleration” force than the 1kg steel ball, resulting in 1/10th the final velocity of the 1kg ball. You can call the resistance to acceleration a “reaction force” if you’d like, but that would just be semantics, as inertia is a very real force. Again, this inertia is just as real in “directional” acceleration as it is in “velocity” acceleration.
Now let’s use the same steel balls for the second example. This time lets drop them from a height of 10 meters. Discounting air resistance, we know that if you release the steel balls at the same time, the steel balls will hit the ground at the same time. We also know that if you remove the air from a glass cylinder and do the same experiment with a feather and a steel ball, they will also both hit the ground at the same time.
Now here’s the remarkable thing about this second example. We know that gravity is an acceleration force, and has an acceleration value of 9.8 m/s. We also know from the first example that the 10kg ball has 10 times the inertia (resistance to acceleration) that the 1kg ball has. The difference in inertia would be even greater between the feather and the steel ball. So why do all the objects hit the ground at the same time, given that the same acceleration is being applied to all the objects, all of which have different masses and inertia? The answer is that the amount of gravitational attraction exerted by each object towards the earth varies directly with its mass. The greater the mass, the greater the gravitational attraction. This means each object will experience the same units of acceleration (velocity change), because the amount of gravitational attraction between each object and the earth will vary based on each object’s mass. So the more inertia an object has, the greater the attraction to overcome that inertia. The differences in attraction MUST be directly proportional to the differences in mass (or inertia) or else the objects could NOT hit the ground at the same time.
In a normal gravity environment where buoyancy is concerned, the denser objects (density expressed as kg/cm^2) will go to the bottom, due to the greater gravitational attraction of the denser objects. In an artificial gravity environment created by a centripetal force, there is no gravitational attraction pulling any of the objects towards the center, since the acceleration around the arc is mechanical and not gravitational. Thus the mass of the object(s) cannot use their innate gravitational attraction to help draw them towards the center of the arc. Instead, the inertia of the objects resists being drawn into travel around an arc, because the “directional” acceleration is turning them away from straight-line travel. The greater the mass, the greater the inertia and the greater the resistance to being drawn into the arc by the “directional” acceleration of the centripetal force. So in this environment, the greater inertia of the denser objects (or material) causes them to settle towards the outside of the arc. That’s how the lab centrifuge works.
BTW, the “centrifugal” force is real, and is nothing more than the inertia of an object resisting the purely “directional” acceleration of the “centripetal” force. Again remember the formula for the centripetal force, which is:
F = m * v^2 / R (F = centripetal force, m = mass, v = velocity, R = radius)
Note the things in the formula that increase the value of the centripetal force. Increasing the mass will increase the force, because of the greater inertia of the greater mass. Increasing the velocity will increase the force squared, since increasing the velocity increases the rate of the “directional” acceleration of the same mass around the arc (more degrees of arc are covered per second). The greater the change rate in direction, the greater the resistance to that change, and this term is squared. To me, nothing more clearly illustrates the effect of inertia on a centripetal/centrifugal system than a velocity change. Decreasing the radius also increases the force, as this also increases the rate of the “directional” acceleration (again, more degrees of arc are covered per second).
P.S. I wish it was easier to write formulas in this forum.
(edited for typos and greater clarity)
[ 31 December 2001: Message edited by: Flight Safety ]</p>
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Flight Safety, that sounds good. A few small points.
<ul type="square">[*]Physicists do define "both" accelerations by calling an acceleration a vector quantity. All you've done by defining two types is you've decomposed acceleration into two independant orthogonal scalar quantities.[*]That Einstein principle that you can't tell if you're in a gravity field is called Equivalence. Lots on the net if you're interested.[*]Centrifugal force is not a real force. You're talking physics, so use physics terminology. It may seem 'real' to you, but that's not what real force means.[/list]
Happy New Year, Everyone.
<ul type="square">[*]Physicists do define "both" accelerations by calling an acceleration a vector quantity. All you've done by defining two types is you've decomposed acceleration into two independant orthogonal scalar quantities.[*]That Einstein principle that you can't tell if you're in a gravity field is called Equivalence. Lots on the net if you're interested.[*]Centrifugal force is not a real force. You're talking physics, so use physics terminology. It may seem 'real' to you, but that's not what real force means.[/list]
Happy New Year, Everyone.
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heedm, I could make 2 or 3 comments regarding your responses, but will focus on only one.
[quote]Centrifugal force is not a real force. You're talking physics, so use physics terminology. It may seem 'real' to you, but that's not what real force means.<hr></blockquote>
If the scientists cannot agree on the issue of whether the "centrifugal" force is real or not, why do I have to use the terminology of only one camp in the debate?
Einstein said in the theory of general relativity, that velocity is relative since there is no absolute fixed frame of reference for speed in the universe, so any frame of reference is relative and valid. This means that it's just as valid to say a car goes past a filling station as to say that a filling station goes past a car, since the filling station is also moving as the earth rotates on its axis and as it orbits around the sun.
So if I park my car on a road with the front end facing a 35,000kg truck traveling at 100km/h and it hits my car, what happens to my car? It seems fair to say that the force of the truck traveling down the road did the damage. But then there's Einstein, so we park the truck on the road and drive the car at 100km/h into the front end of the truck, and now what happens to the car? Did the inertia of the truck exert an "unreal" force on the front end of my car? I think not.
If there had been one of those Hollywood cardboard cutouts of Brittney Spears sitting stationary in the road and my car hit that instead of the truck at 100km/h, don't you think the results would have been different? My question is, what "force" bent the sheetmetal of my car when it hit the stationary truck with its inertia?
As a side note, doesn't "equilibrium" require the balance of 2 opposing forces? For an object to be in rotation at a constant speed and radius from a center in a centripetal/centrifugal system, it MUST be in a state of equilibrium. But an "equilibrium" between what 2 forces?
As I said in my previous post...BTW, the “centrifugal” force is real, and is nothing more than the inertia of an object resisting the purely “directional” acceleration of the “centripetal” force.
(edited for typos and additional clarity)
[ 31 December 2001: Message edited by: Flight Safety ]</p>
[quote]Centrifugal force is not a real force. You're talking physics, so use physics terminology. It may seem 'real' to you, but that's not what real force means.<hr></blockquote>
If the scientists cannot agree on the issue of whether the "centrifugal" force is real or not, why do I have to use the terminology of only one camp in the debate?
Einstein said in the theory of general relativity, that velocity is relative since there is no absolute fixed frame of reference for speed in the universe, so any frame of reference is relative and valid. This means that it's just as valid to say a car goes past a filling station as to say that a filling station goes past a car, since the filling station is also moving as the earth rotates on its axis and as it orbits around the sun.
So if I park my car on a road with the front end facing a 35,000kg truck traveling at 100km/h and it hits my car, what happens to my car? It seems fair to say that the force of the truck traveling down the road did the damage. But then there's Einstein, so we park the truck on the road and drive the car at 100km/h into the front end of the truck, and now what happens to the car? Did the inertia of the truck exert an "unreal" force on the front end of my car? I think not.
If there had been one of those Hollywood cardboard cutouts of Brittney Spears sitting stationary in the road and my car hit that instead of the truck at 100km/h, don't you think the results would have been different? My question is, what "force" bent the sheetmetal of my car when it hit the stationary truck with its inertia?
As a side note, doesn't "equilibrium" require the balance of 2 opposing forces? For an object to be in rotation at a constant speed and radius from a center in a centripetal/centrifugal system, it MUST be in a state of equilibrium. But an "equilibrium" between what 2 forces?
As I said in my previous post...BTW, the “centrifugal” force is real, and is nothing more than the inertia of an object resisting the purely “directional” acceleration of the “centripetal” force.
(edited for typos and additional clarity)
[ 31 December 2001: Message edited by: Flight Safety ]</p>
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Here's an excellent Physics web site. A few of its pages cover the topic of this thread.
<a href="http://www.mcasco.com/p1outln.html" target="_blank">http://www.mcasco.com/p1outln.html</a>
________
How's this for a neat statement, related to unification theory?
"Gravity is matter's memory that it once was light."
<a href="http://www.mcasco.com/p1outln.html" target="_blank">http://www.mcasco.com/p1outln.html</a>
________
How's this for a neat statement, related to unification theory?
"Gravity is matter's memory that it once was light."
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Flight Safety, it's not that scientists don't agree whether centrifugal force is real or apparent, it's that some don't care or understand. You don't have to use the terminology of the physics "camp" in this debate. Which "camp" are you in? What is your definition of real vs apparent with respect to forces? What term do you use to distinguish what I call real and apparent forces?
Be careful in quoting a small part of what Einstein or anyone has said. According to your reference frame reasoning distant planets travel many times the speed of light. How? The earth is stationary so those stars that orbit about the earth travel the circumference of that orbit once every 24 hours. Pick one far enough and you have a very fast planet.
Point is, that reference frame thing works as long as you don't accelerate the reference frame. A rotating reference frame has accelerating components.
Equilibrium means there is no net force. Something moving in a curved path requires a force to move it in that path, thus it is not in equilibrium.
Be careful in quoting a small part of what Einstein or anyone has said. According to your reference frame reasoning distant planets travel many times the speed of light. How? The earth is stationary so those stars that orbit about the earth travel the circumference of that orbit once every 24 hours. Pick one far enough and you have a very fast planet.
Point is, that reference frame thing works as long as you don't accelerate the reference frame. A rotating reference frame has accelerating components.
Equilibrium means there is no net force. Something moving in a curved path requires a force to move it in that path, thus it is not in equilibrium.
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Boy, I just can't seem to swat all of the flies here can I? Somehow I ended up in a camp of scientists who either don't know or don't care about real or apparent forces. Somehow I got the distant planets moving at several multiplies of the speed of light in the far reaches of the universe. How, oh how, did this happen?
Anyway, I found this definition of "apparent forces" in the Harcourt Academic Press Dictionary of Science and Technology.
apparent force Mechanics. a fictitious force that appears to exist from physical experience or observation made in a noninertial frame of reference; e.g., the fact that a force seems to pull passengers forward if a car stops suddenly, or pull them outward as the car rounds a curve.
Is it just me, or is this definition absolutely laughable?
Imagine you're in a 4 door car sitting in the back seat next to the right rear door. The driver suddendly turns left at a round-about and you are pressed against the door. Assume the door in not closed well, and the pressure suddenly opens the door forward (assuming the door's hinges are on the pillar between the doors) and you tumble out onto the pavement.
My gosh, this "apparent force" actually did some work, since it opened the car door by moving it forward on its hinges. Can you hear yourself telling the ambulance driver that an imaginary ficticious "apparent force" dumped you out of the car onto the pavement? Suppose a car you're riding in as a front seat passenger, is stopped very suddenly during straight line travel, what "apparent force" smashed your head against the windshield? Those seat belts appear awefully strong to me for such an imaginary force. Oops, but that's "apparent" isn't it, as the seat belts only "look" that strong.
BTW, how on earth does traveling in a car place me in a "noninertial" frame of reference?
Absolutely bewildering.
(edited for typos)
[ 31 December 2001: Message edited by: Flight Safety ]</p>
Anyway, I found this definition of "apparent forces" in the Harcourt Academic Press Dictionary of Science and Technology.
apparent force Mechanics. a fictitious force that appears to exist from physical experience or observation made in a noninertial frame of reference; e.g., the fact that a force seems to pull passengers forward if a car stops suddenly, or pull them outward as the car rounds a curve.
Is it just me, or is this definition absolutely laughable?
Imagine you're in a 4 door car sitting in the back seat next to the right rear door. The driver suddendly turns left at a round-about and you are pressed against the door. Assume the door in not closed well, and the pressure suddenly opens the door forward (assuming the door's hinges are on the pillar between the doors) and you tumble out onto the pavement.
My gosh, this "apparent force" actually did some work, since it opened the car door by moving it forward on its hinges. Can you hear yourself telling the ambulance driver that an imaginary ficticious "apparent force" dumped you out of the car onto the pavement? Suppose a car you're riding in as a front seat passenger, is stopped very suddenly during straight line travel, what "apparent force" smashed your head against the windshield? Those seat belts appear awefully strong to me for such an imaginary force. Oops, but that's "apparent" isn't it, as the seat belts only "look" that strong.
BTW, how on earth does traveling in a car place me in a "noninertial" frame of reference?
Absolutely bewildering.
(edited for typos)
[ 31 December 2001: Message edited by: Flight Safety ]</p>
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Your research is paying off! The definition of apparant force is perfectly correct.
It is important for the purest to recognize the difference between an applied force and a 'force" that is really due to an acceleration. The truck hits your car, which accelerates it, and the passengers, formerly at rest, are accelerated and "feel" a "force" that is their inertial resistance to the acceleration.
Similarly, the velocity is a vector, with both direction and magnitide (speed) as its constituents. One changes velocity (accelerates) when at a constant speed while circling. This acceleration is "felt" as a force while in the turning/accelerating reference frame, but is seen as a non-force in the stationary outside reference frame. The distinction is important when looking at a fresh problem, where an engineer must decide how to design for the applied loads. By convention and by definition, he draws a "free body diagram" which defines all the forces on the free body in an UNACCELERATED state (an inertial reference frame).
This seems to a lay person as a minor point, but it took western science about 2000 years to get there. Read Galileo's 'Dialogs" which are wonderfully easy and clear, where he for the first time determined the distinction. Archimedes believed (as did the whole human race) that a natural retarding "force" existed in nature, until Galileo realized the inertial reference frame concept. Thus did he drop the cannon balls off the Leaning Tower (at least in legend) to demonstrate one part of his unified inertial reference frame concept.
This thread could go on for a very long time, and we all get sharper because of it!
Your research is paying off! The definition of apparant force is perfectly correct.
It is important for the purest to recognize the difference between an applied force and a 'force" that is really due to an acceleration. The truck hits your car, which accelerates it, and the passengers, formerly at rest, are accelerated and "feel" a "force" that is their inertial resistance to the acceleration.
Similarly, the velocity is a vector, with both direction and magnitide (speed) as its constituents. One changes velocity (accelerates) when at a constant speed while circling. This acceleration is "felt" as a force while in the turning/accelerating reference frame, but is seen as a non-force in the stationary outside reference frame. The distinction is important when looking at a fresh problem, where an engineer must decide how to design for the applied loads. By convention and by definition, he draws a "free body diagram" which defines all the forces on the free body in an UNACCELERATED state (an inertial reference frame).
This seems to a lay person as a minor point, but it took western science about 2000 years to get there. Read Galileo's 'Dialogs" which are wonderfully easy and clear, where he for the first time determined the distinction. Archimedes believed (as did the whole human race) that a natural retarding "force" existed in nature, until Galileo realized the inertial reference frame concept. Thus did he drop the cannon balls off the Leaning Tower (at least in legend) to demonstrate one part of his unified inertial reference frame concept.
This thread could go on for a very long time, and we all get sharper because of it!
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Flight Safety
I am going through the previously mentioned Physics course and it is very easy reading.
You will find centripetal force is explained by applying your vector arithmetic to a circumference and looking at smaller and smaller time intervals between adjacent velocity vectors.
<a href="http://www.mcasco.com/p1mot2d.html" target="_blank">Motion in 2 Dimensions</a>
Your concern about 'inertial reference frame' is defined in <a href="http://www.mcasco.com/p1ns.html" target="_blank"> The Nature of Space</a>
_________
Just wait until the topic changes to the Coriolis EFFECT. Or would that be a hijacking of this thread? <img src="smile.gif" border="0">
[ 31 December 2001: Message edited by: Dave Jackson ]</p>
I am going through the previously mentioned Physics course and it is very easy reading.
You will find centripetal force is explained by applying your vector arithmetic to a circumference and looking at smaller and smaller time intervals between adjacent velocity vectors.
<a href="http://www.mcasco.com/p1mot2d.html" target="_blank">Motion in 2 Dimensions</a>
Your concern about 'inertial reference frame' is defined in <a href="http://www.mcasco.com/p1ns.html" target="_blank"> The Nature of Space</a>
_________
Just wait until the topic changes to the Coriolis EFFECT. Or would that be a hijacking of this thread? <img src="smile.gif" border="0">
[ 31 December 2001: Message edited by: Dave Jackson ]</p>
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After reading the various postings on this thread and many of the sub tier documents referenced in some of the postings I now have an understanding of why I majored in Industrial Design instead of Engineering. Although I do not possess an Engineering degree I have a detailed understanding of space craft, weightlessness, rocket engines, ballistic missiles, aircraft systems and helicopter systems as well as ships and other military systems and systems used in civil applications. True, I don’t fully understand the detailed engineering that goes into the design of some of these systems but I do have an appreciation of those engineering requirements and I have enough of an understanding relative to how these systems were designed to work and those elements that keep them from performing according to specs. So, based on that understanding of my limitations I will cease adding to the confusion and the possibility of highjacking a thread unless I am driven to it.
[ 01 January 2002: Message edited by: Lu Zuckerman ]</p>
[ 01 January 2002: Message edited by: Lu Zuckerman ]</p>
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Flight Safety, I wasn't meaning to imply that you were one of those who don't care or don't know. That was just an explanation of why not all scientists are on the same track.
The definition you quoted is not completely laughable. I don't agree with the word "fictitious" (perhaps that part is laughable).
The presssure of your body doesn't necessarily swing the door open, it just adds enough load to the latch to allow the door to travel in a straight line. Put your seatbelt on and without pushing at all on the door, open the latch. You'll find that the door wants to swing open without the pressure from your body against it (car is turning away from that door).
As far as the head smashed into the windshield. I don't consider any apparent forces being created there. Your head and the windsheild have different speeds. When they meet they collide. No forces required. It's the same as shooting a bullet through the windsheild from a distance. The force that accelerates the bullet stops acting on the bullet shortly after it leaves the barrel, yet guns are effective at distances that go well beyond their barrels.
Travelling in a car puts you in a non inertial reference frame when the car is accelerating. Accelerating includes changing the speed and changing the heading. If you're stationary on the ground, you're in a non-inertial reference frame. That's how the coriolis force that causes air to circulate lows rather than head toward them is explained.
You seem to understand most of this material. Is it just the terminology of the apparent vs real that you don't agree with?
The definition you quoted is not completely laughable. I don't agree with the word "fictitious" (perhaps that part is laughable).
The presssure of your body doesn't necessarily swing the door open, it just adds enough load to the latch to allow the door to travel in a straight line. Put your seatbelt on and without pushing at all on the door, open the latch. You'll find that the door wants to swing open without the pressure from your body against it (car is turning away from that door).
As far as the head smashed into the windshield. I don't consider any apparent forces being created there. Your head and the windsheild have different speeds. When they meet they collide. No forces required. It's the same as shooting a bullet through the windsheild from a distance. The force that accelerates the bullet stops acting on the bullet shortly after it leaves the barrel, yet guns are effective at distances that go well beyond their barrels.
Travelling in a car puts you in a non inertial reference frame when the car is accelerating. Accelerating includes changing the speed and changing the heading. If you're stationary on the ground, you're in a non-inertial reference frame. That's how the coriolis force that causes air to circulate lows rather than head toward them is explained.
You seem to understand most of this material. Is it just the terminology of the apparent vs real that you don't agree with?
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heedm, I anticipated your response regarding the car door and would like to make the following amendment to the example. Suppose the car were in a .5g left turn (around the round-about), and the door had a spring on the hinge that JUST held it closed against the .5g force. With the example being the same otherwise, I think the man pressed against the door would still open it, and his enertia would have done the work.
The interesting thing about this modification, it that it presents a rather crude way of isolating and studying 2 different "forces" acting on the door. Those "forces" are the enertia of the door itself and its reaction to the centripetal force of the car turning, and the enertia of the man sitting in the back seat leaning against the door, both of which are "apparent" centrifugal forces.
That has lead me to contemplating "inertial" and "non-inertial" reference frames, free body diagrams, and "apparent" vs real forces. All of these appear related and it seems that these things are used by scientists to facilitate the breakdown and study of all the individual "forces" acting on moving objects under scrutiny. In other words they seem to provide isolated "perspectives" focused on the particular forces being examined, I think.
I generally understand "apparent forces" and the Coriolis effect, but I don't understand what seems to me to be a very lose use of the word "apparent". I'll have to cogitate on these things for a while and get back.
BTW, this particular thread and the debate contained within it has been great fun.
The interesting thing about this modification, it that it presents a rather crude way of isolating and studying 2 different "forces" acting on the door. Those "forces" are the enertia of the door itself and its reaction to the centripetal force of the car turning, and the enertia of the man sitting in the back seat leaning against the door, both of which are "apparent" centrifugal forces.
That has lead me to contemplating "inertial" and "non-inertial" reference frames, free body diagrams, and "apparent" vs real forces. All of these appear related and it seems that these things are used by scientists to facilitate the breakdown and study of all the individual "forces" acting on moving objects under scrutiny. In other words they seem to provide isolated "perspectives" focused on the particular forces being examined, I think.
I generally understand "apparent forces" and the Coriolis effect, but I don't understand what seems to me to be a very lose use of the word "apparent". I'll have to cogitate on these things for a while and get back.
BTW, this particular thread and the debate contained within it has been great fun.
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Lu, I enjoy your contributions and unique perspective to these questions. Lurking is fine, but if you have a question that you don't want to post, just email me.
Flight Safety, that sounds good. Another way of looking at the .5g door example is that the mass of the man is added to the mass of the door, so the force on the spring is greater. Actually it's the same way that you explained it, I just find this one a bit easier to picture.
You said, "BTW, this particular thread and the debate contained within it has been great fun."
Yes, same for me. That's why I participate.
Happy New Year!
[ 01 January 2002: Message edited by: heedm ]</p>
Flight Safety, that sounds good. Another way of looking at the .5g door example is that the mass of the man is added to the mass of the door, so the force on the spring is greater. Actually it's the same way that you explained it, I just find this one a bit easier to picture.
You said, "BTW, this particular thread and the debate contained within it has been great fun."
Yes, same for me. That's why I participate.
Happy New Year!
[ 01 January 2002: Message edited by: heedm ]</p>
I am enjoying this too. Dont leave Lu.
Since I posted at Christmas, there has been even more examples to add to the list. Here are the current examples people have used to try and explain/question the same thing:
1. David with his rock in a sling Vs Goliath.
2. A girl holding a chain on a merry go round.
3. A marble on a rotating disc.
4. An Alcohol pump.
5. Droop stops on a rotor disc.
6. A car going around a round about.
7. A steel ball on a chain.
8. A tractor on a chain around a stump.
9. Liquid dye on a rotating disc.
10. Sattelites and orbits.
The new ones:
11. A dairy farmer's centrifuge.
12. a science lab centrifuge.
13. NASA & USAF centrifuges.
14. a man on a cornering bus holding a helium balloon.
15. a pilot's bodily fluids.
16. a jar full of water.
17. ANOTHER cornering car.
18. a 35,000 Kg truck and cardboard Britteny Spears cuttouts.
C'mon guys, lets get to at least 20.....
Since I posted at Christmas, there has been even more examples to add to the list. Here are the current examples people have used to try and explain/question the same thing:
1. David with his rock in a sling Vs Goliath.
2. A girl holding a chain on a merry go round.
3. A marble on a rotating disc.
4. An Alcohol pump.
5. Droop stops on a rotor disc.
6. A car going around a round about.
7. A steel ball on a chain.
8. A tractor on a chain around a stump.
9. Liquid dye on a rotating disc.
10. Sattelites and orbits.
The new ones:
11. A dairy farmer's centrifuge.
12. a science lab centrifuge.
13. NASA & USAF centrifuges.
14. a man on a cornering bus holding a helium balloon.
15. a pilot's bodily fluids.
16. a jar full of water.
17. ANOTHER cornering car.
18. a 35,000 Kg truck and cardboard Britteny Spears cuttouts.
C'mon guys, lets get to at least 20.....
To Flight Safety:
In relation to your man falling out the door of the cornering car, I believe we have discussed this exact point on page 2 of this thread with your introduced example of the man in a van on rollerskates. I had provided an answer at that time, and will be happy to re paste it here if you require. We were discussing points 13 and 14 of my logic chain on centripetal that explains your question, though I note you chose not to respond to any part of my explanation (is it because I interupted you?).
At the end of the day, centrifugal is only as real as the force you feel pushing you backwards when your car accelerates in a straight line - I.E. it is "felt" and seems real, I.E. it is "apparant" not "real".
...but then why does a man whom is running along with his dog on a leash.....
In relation to your man falling out the door of the cornering car, I believe we have discussed this exact point on page 2 of this thread with your introduced example of the man in a van on rollerskates. I had provided an answer at that time, and will be happy to re paste it here if you require. We were discussing points 13 and 14 of my logic chain on centripetal that explains your question, though I note you chose not to respond to any part of my explanation (is it because I interupted you?).
At the end of the day, centrifugal is only as real as the force you feel pushing you backwards when your car accelerates in a straight line - I.E. it is "felt" and seems real, I.E. it is "apparant" not "real".
...but then why does a man whom is running along with his dog on a leash.....
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The following was taken from a paper presented on the Internet by Nick Lappos regarding blade lag phenomenon. It should be noted that in his presentation he made a disclaimer but he still alluded to centrifugal force in making his point.
“Why doesn't the blade just lag back against the rear stops when we increase
pitch (and therefore drag)? Because the centrifugal force (physics jocks cut
me some centripetal slack, OK?) tries to make the blade stay at 90 degrees to
the hub, and that force is awesome. Typical CF for an S-76 is 33,000 pounds.
To get the blade to lag 3 degrees back, the CF will fight with a restoring
lead force of 1700 pounds”.
Although centrifugal force is not a real force according to some physicists it must be addressed at times even if only to make a point to those of us that do not have an engineering background.
[ 04 January 2002: Message edited by: Lu Zuckerman ]</p>
“Why doesn't the blade just lag back against the rear stops when we increase
pitch (and therefore drag)? Because the centrifugal force (physics jocks cut
me some centripetal slack, OK?) tries to make the blade stay at 90 degrees to
the hub, and that force is awesome. Typical CF for an S-76 is 33,000 pounds.
To get the blade to lag 3 degrees back, the CF will fight with a restoring
lead force of 1700 pounds”.
Although centrifugal force is not a real force according to some physicists it must be addressed at times even if only to make a point to those of us that do not have an engineering background.
[ 04 January 2002: Message edited by: Lu Zuckerman ]</p>