M lies on the horizontal turntable of a record player.
As the turntable starts rotating with increasing rotational speed, the coin rotates with the turntable, remaining at rest relative to it. Suppose that at a certain time the angular velocity of the turntable and coin is w , the coin is a distance r of the center of rotation, and the angular acceleration of the two is a . Find the direction and magnitude of the frictional force exerted by the turntable on the coin at this time. The coefficient of static friction between the coin and the turntable is m . Your expression may include M, w , r, a , and m .