A block of mass 15 kg is placed on a long trolley. The coefficient of static friction between the block and the trolley is 0.18. The trolley accelerates from rest with` 0.5 m s^(-2)` for 20 s and then moves with uniform velocity. Discuss the motion of the block as viewed by (a) a stationary observer on the ground, (b) an observer moving with the trolley.

Explanation

(a) Maximum frictional force possible for opposing impending relative motion between the block and the trolley `= 150 xx 0.18 = 27 N`, which is more than the frictional force of `15 xx 0.5 = 7.5 N ` needed to accelerate the box with the trolley. When the trolley moves with uniform velocity, there is no force of friction acting on the block.
(b) For the accelerated (non-inertial) observer, frictional force is opposed by the pseudo- force of the same magnitude, keeping the box at rest relative to the observer. When the trolley moves with uniform velocity there is no pseudo-force for the moving (inertial) observer and no friction.