A particle, P, of mass 4 kg is resting on a rough, horizontal table. A light, elastic string, of natural length 2 in and modulus of elasticity 50 N, is attached to P. It is then passed over a smooth pulley to a second particle, Q, of mass 3 kg. Q is hanging freely below the table and the system is on the point of slipping. Determine the coefficient of friction and find the extension of the string.