Figure (a) and (b) shows two different arrangements of a spool being pulled with a constant force `F` in each case. Identify the direction of motion of the centre of mass, and the direction of friction force in each case.

a. The `CM` moves along……………….`x`-axis .
b. The friction force acts along…………………. `x`-axis.
c. The `CM` moves along .....`x`-axis.
d. The friction force acts along…………….. `x`-axis.

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.