Question

A coin is placed on a horizontal platform, which undergoes horizontal simple harmonic motion about a mean position O. The coin does not slip on the platform. The force of friction acting on the coin is F. 

(a) F is always directed towards O. 

(b) F is directed towards O when the coin is moving away from O, and away from O when the coin moves towards O. 

(c) F = 0 when the coin and platform come to rest momentarily at the extreme position of the harmonic motion. 

(d) F is maximum when the coin and platform come to rest momentarily at the extreme position of the harmonic motion.

Answer 1

Correct Answer is: (a, d)

The only horizontal force acting on the coin is the force of friction F. Therefore, its horizontal acceleration is always in the direction of F, and its magnitude is F/m. The magnitude and the direction of F can thus be obtained from the magnitude and the direction of the acceleration.