Assume that a coin is placed on circular disk and now a disk is rotated with constant angular velocity.

If there is no friction between the surfaces of a disk and coin, according to theory the coin will move away from centre of disk. But I have confusion here that the centripetal and centrifugal forces are of equal magnitude so why latter comes to play effectively?


The coin will not move.

First, to differentiate between centrifugal and centripetal, I'll start by stating the definitions first.

Centrifugal force is the apparent force that draws a rotating body away from the center of rotation. It is caused by the inertia of the body as the body's path is continually redirected.

Centripetal force is a force that makes a body follow a curved path: its direction is always orthogonal to the velocity of the body, toward the fixed point of the instantaneous center of curvature of the path. Centripetal force is generally the cause of circular motion.

This means centrifugal force is a fictitious force (or pseudo force). It isn't actually there. Any body travelling in a straight tries to resist its direction from being changed (inertia). When a body moves in a circle, it tries to go straight at every point on the circumference. It is centripetal force which makes the body keep rotating by pulling it back to the circular trajectory. So centripetal force is basically the cause of circular motion. That is why when you rotate anything over your head and release it, it goes straight, as a tangent to the circle.

Coming back to the question, friction itself will act as a centripetal force in this case. If there is no friction, there is no centripetal force and hence, there is no centrifugal force. Therefore, the coin does not move.

  • $\begingroup$ You are welcome nikhil. See the edited answer. If your problem is solved, please accept the answer. $\endgroup$ – user42733 Apr 2 '14 at 16:52

Here, because the coin is placed at the center, the centrifugal forces balance each other. Every point mass in the coin has it's conjugate point at the diameter passing through it and on the same distance from the center on the other side.

Hence the coin is under equilibrium and does not fly off.


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