You see a penny sitting on a turntable rotating with constant speed. How many forces are acting on the penny? a. The friction force of the turntable on the penny pointing inward towards the center.
b. The downward force of the earth on the penny and the upward force of the turntable on the penny.
c. The centripetal force.
d. The downward force of the earth on the penny, the upward force of the turntable on the penny. and the inward friction force of the turntable on the penny.
e. The downward force of the earth on the penny.

d.

Explanation:

Since the penny is not accelerated in the vertical direction, there must be two forces (equal and opposite each other) acting on it in this direction.One of them is the downward force that the earth exerts on the penny, the other one is the normal force exerted by the turntable on the penny.In order to keep the penny rotating, there must be a third force, the centripetal force.This centripetal force is not a new type of force, it's the net force acting  on the penny inward.This force is provided by the friction force of the turntable on the penny, as the turntable is rotating at a constant speed, so no tangential acceleration exists.