A block P of mass m is placed on a frictionless horizontal surface. Another block Q of same mass is kept on P and connected to the wall with the help of a spring of spring constant k as shown in the figure, µs is the coefficient of friction between P
and Q. The blocks move together performing SHM of amplitude A. The maximum value of the friction force between P and Q is
Let Q is displaced through a small distance x. Now force exerted by spring on Q is k x If the initial acceleration of the system be A, then
From eq. (2), f =(m k x /2 m) = (k x /2).