A bullet losses $1 {\mathrm{n}}^{-1}$ of its velocity in passing through a plank. What is the least number of planks required to stop the bullet ? (Assuming constant retardation)

Calculate the acceleration of the system, tension in the string and thrust on the pulley in terms of g for the situation shown in following diagram.

Two blocks of masses 8kg and 4 kg respectively are connected by a string as shown. Calculate their accelerations if they are initially at rest on the floor, after a force of 100N is applied on the pulley in the upward direction (g = 10 m/s²)