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I have newly begun to study about the role of friction in circular motion and it has beginning to confuse me that when does friction act as a centripetal force and when does it act as a tangential force.

I know that in the case of a car in circular motion, the friction acts a centrifugal force as the car has the tendency to skid from the path if the friction is absent.

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However if we keep a collar/ring, on a circular path and move it, the friction acts tangentially backwards.

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My argument as to why this happens: The collar/ring is constrained to move in a circular path, therefore it does not need frictional force to act as a centripetal force to keep it moving in the circle. Hence, in this case the component on Normal force acting on the plane of the circular path acts as the centripetal force here instead of friction.

Is my argument correct?

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  • $\begingroup$ I know that in the case of a car in circular motion, the friction acts a centrifugal force as the car has the tendency to skid from the path if the friction is absent Centrifugal ? Are you sure ??? If it's centrifugal, then in the case of friction car should skid to the outside sooner than without friction, right ? Maybe you mean centripetal instead ? $\endgroup$ Mar 5 at 9:33

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Friction always acts in the opposite direction of the current or impending motion between two surfaces. You are correct that in the case of the collar/ring that the normal force between the ring and the collar acts as the centripetal force.

In the case of the car, a full analysis is more complicated. But from a simple perspective, the $net$ force acting at the car's center of mass must equal the required centripetal force to maintain circular motion. This net force will be the sum of the $individual$ friction forces acting on each tire. Locally, the friction force on a given tire will act in the opposite direction of the impending motion between that tire and the ground. This will be a function of the orientation of the tires on the front axle.

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