A highly rigid cubical block A of small mass M and side L is fixed rigidly on the other cubical block of same dimensions and of low modulus of rigidity η such that the lower face of A competely covers the upper face of B. The lower face of B is rigidly held on a horizontal surface. A small force F is applied perpendicular to one of the side faces of A. After the force is withdrawn, block A executes small oscillations, the time period of which is given by