question_answer 34)

A block of mass 15 kg is placed on a long trolley. The coefficient of friction between the block and the trolley is 0.18. The trolley accelerates from rest with  for 20 s and then moves with uniform velocity. Discuss the motion of the block as viewed by (i) a stationary observer on the ground (ii) an observer moving with the trolley.  

Answer:

Mass of block, m = 15 kg   Maximum value of static friction,   Force acting on the block during the accelerated motion,   As  so the block does not move. It remains at rest w.r.t. the trolley, even when it is accelerated. When the trolley moves with uniform velocity, acceleration is zero and hence no force is acting on the trolley. (i) The stationary observer will see the accelerated and the uniform motions. (ii) When the observer is in the trolley, he is in an accelerated or non-inertial frame. The laws of motion are not applicable. But during uniform motion he will see that the block is at rest w.r.t. him.