A highly rigid cubical block $A$ of small mass $M$ and side $L$ is fixed rigidly onto another cubical block $B$ of same dimensions and of low modulus of rigidly $\eta$such that lower lower face of $A$ completely 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 face of $A$. After the force is withdrawn, block $A$ executes small oscillations, the time period of which is given by.