centripetal force vs centrifugal force.

[casrip1@gmx.com]

hey, my teacher says that centripetal force is the real force and centrifugal force doesn't really exist. whereas my philosophy is quite the opposite.

 

we argue that an object travelling in a circle experiences an acceleration towards the center of the circle. but i find it hard to wrap my head around that concept. if you were on the curved ramp of a highway entrance, you don't feel that centripetal force, you feel that centrifugal force, you being pulled out and away from the circle. the earth spinning in a circle doesn't increase the gravity we feel pulling on us, it decreases it. we experience acceleration away from the center, aka the centrifugal acceleration. so why is it that we argue that centripetal force is the real one and centrifugal is the "fictitious force" when we feel centrifugal force yet never actually experience the centripetal force first hand.

Edited April 13, 2013 by casrip1@gmx.com

[elfmotat]

The centripetal force is the force that keeps a rotating object in orbit. For example, if you were to tie a ball to the end of a rope and whirl it around, the tension in the rope would be the centripetal force.

 

The centrifugal force is what is known as a "fictitious force." A fictitious force is one that arises because the observer is in a non-inertial reference frame. Fictitious forces occur because the reference frame itself is accelerating. If you were to be put inside a giant elevator that moves up with constant acceleration, you would feel a "force" pulling you to the ground. What's "really" happening is that the floor of the elevator is accelerating up towards your feet, though from inside the elevator it seems like some invisible mysterious force is pulling you down.

 

Similarly, if you were to be placed inside a rotating spaceship then you would observe a mysterious force pulling you to the walls. What's "really" happening is that the walls are pushing on your feet, keeping you in orbit. I.e. they're applying a centripetal force. But from inside the ship, all you can tell is that you're being pressed to the walls with a force of magnitude [math]m \omega^2 r[/math].

[mathematic]

The point that has to be made is that "centrifugal force" is simply inertia. In other words, if it weren't for the centripetal force, you would continue in a straight line at constant speed.

[rwjefferson]

The centripetal force is the force that keeps a rotating object in orbit.

 

And here I thought that force was called gravity. Oh... right. According to relativity dogma, gravity is not really a force.

 

in truth

ron

[Delta1212]

 

And here I thought that force was called gravity. Oh... right. According to relativity dogma, gravity is not really a force.

 

in truth

ron

Tie a string to a rock and spin in circles. The rock will orbit you (assuming you're strong enough to get it off the ground, of course). Gravity has nothing to do with keeping it in that orbit.

[D H]

if you were on the curved ramp of a highway entrance, you don't feel that centripetal force, you feel that centrifugal force, you being pulled out and away from the circle.

You feel pushed in toward the center, not away.

 

Suppose you go around that corner a bit to fast, so fast that the door presses against you. The force that the door exerts on you is inward (centripetal), not outward (centrifugal). Now let's look at things from the perspective of a frame of reference fixed on you. From this perspective, you aren't moving. It's the road and the landscape that are moving from this perspective. Since you aren't moving the net force on you must be zero -- assuming Newton's laws of motion still apply. You can feel that force exerted by the door on you. That's a real force that can be measured. If Newton's laws still apply, that means there must be some other force you cannot feel that counteracts out this force that you can feel.

 

This is the fictitious centrifugal force, and it's solely a result of assuming that Newton's laws still apply in this non-inertial frame of reference. Get rid of that assumption and you get rid of the centrifugal force. Fictitious forces such as the centrifugal force are a convenient fiction for extending Newton's first two laws of motion to a regime where they don't apply. One downside of this fiction is that you have to throw Newton's third law out. Newton's third law applies to real forces, but not to fictitious forces.

[elfmotat]

One downside of this fiction is that you have to throw Newton's third law out. Newton's third law applies to real forces, but not to fictitious forces.

 

Could you give me an example of where the third law falls apart? Because I'm having trouble thinking of one.

[D H]

Could you give me an example of where the third law falls apart? Because I'm having trouble thinking of one.

The third law falls apart for fictitious forces. There is no third law counterpart to the centrifugal force, or to any other fictitious force. A real force, one that is subject to Newton's third law, is the same in all frames of reference. Fictitious forces on the other hand are frame-dependent. They're a fiction (a very convenient fiction) that results from force-fitting Newton's laws to a domain where they don't apply.

[rktpro]

Centripetal force is a class of force. Gravity,tension and friction etc. can act as centripetal force. And when you are on a circular road you feel pushed away not in as you experience force in your frame and it is non inertial.

[rwjefferson]

geometric models

'For a satellite in orbit around a planet, the centripetal force is supplied by gravity. Some sources, including Newton, refer to the entire force as a centripetal force, even for eccentric orbits, for which gravity is not aligned with the direction to the center of curvature.'

Theo Koupelis (2010). In Quest of the Universe (6th ed.). Jones & Bartlett Learning. p. 83. ISBN 978-0-7637-6858-4

Centripetal force is model for attraction - like tension and gravity. Centrifugal force is a model for resistance to attraction - like inertia. Science evolves as good geometric models are replaced by better, by more fluent models.

ron

ode to urban.viii

warnings from you are points of honor