A block of mass m is at rest relative to the stationary wedge of mass M. The coefficient of friction between block and wedge is `mu`. The wedge is now pulled horizontally with acceleration a as shown in figure. Then the minimum magnitude of a for the friction between block and wedge to be zero is:

Explanation

a. The `CM` moves along the positive `x`-axis because the rest torque about the point of contact is clockwise.
b. The friction force acts along the negative `x` axis. In the absence of friction force, point `O` has a tendency to move along the positive `x`-axis.

c. The `CM` moves along the negative `x` axis because the at torque about the point o contact is anticlockwise.
d. The friction force acts along the negative `x`-axis. No other force is acting along the `x`-axis that can acceleration the centre of mass along the negative `x`-axis.