Three identical metal balls each of radius ' $r$ ' are placed such that an equilateral triangle is formed when centres of three ball are joined. The centre of mass of the system is located at

In case of system of two-particles of different masses, the centre of mass lies

A particle of mass m collides with another stationary particle of mass $M$. The particle $m$ stops just after collision. The coefficient of restitution is

1000 small balls, each weighing 1 gram, strike one square cm of area per second with a velocity $50 \mathrm{~m} / \mathrm{s}$ in a normal direction and rebound with the same velocity. The value of pressure on the surface will be