If fig four identical spheres of equal mass m are suspended by wires of equal length /, so that all spheres are almost touching to each other. If the sphere 1 is released from the horizontal position andall collisions are elastic, the velocity of sphere 4 iust after collision is

. When the sphere 1 is released from horizontal position, then potential energy at height / = kinetic energy at bottomor mgl=12mv2 v=(2gl)Now, the velocity of sphere 1 is transforred to sphere 2 felastic collision), then from 2 to sphere 3 and finally from sphere 3 to sphere 4 Therefore, the sphere 4 attains "  velocity  (2gl)