# Centripetal force in frame of reference of body moving In a circle

Suppose a body is moving in a circle about a fixed point. In the frame of reference of the body, is the centripetal force felt or is only the centrifugal force felt?

More generally, does a body only feel the effect of pseudo forces in an accelerated reference frame?

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Related: physics.stackexchange.com/q/8891/2451 and links therein. – Qmechanic Dec 8 '13 at 20:19

In the frame of reference of the body, is the centripetal force felt or is only the centrifugal force felt?

It depends on what you mean exactly. Consider, for example, the amusement park ride Dumbo at Disneyland:

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On this ride, passengers sit in mini Dumbo replicas and are swung around in a circle. What forces do they feel? Well, firstly, they feel a centrifugal force radially outward. But this is not all. If that were the only force they felt, then in the frame that is stationary with respect to Dumbo, they would accelerate radially outward. Instead, they also feel a normal force of Dumbo pushing them inward that is precisely equal to the centrifugal force, and as a result, as measured in the Dumbo frame, they remain stationary with respect to Dumbo.

Now, we know that if we were to analyze the same situation from the frame of reference of a person watching the ride from the ground, then we would say that there is only one force on the passengers, namely the normal force of Dumbo on them, and this force causes the passengers to accelerate, namely to move in a circle. As a result, the convention is to call the normal force the "centripetal" force. I personally think this is terrible terminology that confuses students because it leads them to believe that "centripetal force" is somehow an independent thing that doesn't need to be comprised of real physical interactions with objects...by anywho.

Now, going back to the accelerated frame, we had noticed that there were two forces acting on the passengers, the (fictitious) centrifugal force, and the normal force. Would you now call the normal force a "centripetal"? If we're doing the analysis in the accelerating frame, then that would be extremely non-standard because in that frame, no circular motion is occurring.

does a body only feel the effect of pseudo forces in an accelerated reference frame?

No! Just look at the above example! The passengers feel the centrifugal force, but they also feel a normal force due to their interaction with dumbo! In general, there can be all sorts of forces that an object feels in an accelerated frame that are not pseudo forces like friction, gravitational forces, electromagnetic forces etc.

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So in a non-inertial radial reference frame, you have to add a fictitious centrifugal force to balance out the centripetal force so the object stays stationary form your radial viewpoint? However, if this is the case then (let's say you are on one of those circular spinny rides at an amusement park) these two forces should balance out so you should not feel any "push" against you from any side. Yet clearly, you do feel an outward push against you. What am I missing? – 1110101001 Jun 21 '15 at 2:08
@1110101001 It seems you're making a common mistake -- a net force of zero does not mean you don't feel anything. As another example, suppose Alice is pushing you toward the left, and Bob is pushing you toward the right with a force of equal magnitude. Those to forces sum to zero, but you feel each of them. – joshphysics Jun 21 '15 at 6:06

In the frame of reference of the body, is the centripetal force felt or is only the centrifugal force felt?

In the frame of reference of body both centripetal and centrifugal forces are felt.

does a body only feel the effect of pseudo forces in an accelerated reference frame?

No the body feels all Pseudo forces and real forces in an accelerated frame of reference.

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-1: Your answer to the second question is wrong. There can be all sorts of forces acting on a body in an accelerated frame beside pseudo forces (e.g. gravity). – joshphysics Dec 8 '13 at 19:35
Once you've edited your answer to address comments, you may want to ping e.g. @joshphysics to see what they think about their downvote. Being polite and open to constructive criticism when doing so will always help, of course. – Emilio Pisanty Mar 14 '14 at 12:37
@EmilioPisanty: You are technically wrong here. josh has already taken back his downvote. The present downvote on the answer is not by joshphysics. I did ping him and he took his downvote back a long time ago. I have deleted my comment in which I pinged him. – user Mar 14 '14 at 14:40