એક ઘર્ષણરહિત ટેબલ પર બ્લોક B છે અને તણા પર

There is no friction between the body $B$ and surface of the table. If the body $B$ is pulled with force $F$ then $F = ({m_A} + {m_B})\,a$

Due to this force upper body $A$ will feel the pseudo force in a backward direction.

$f = {m_A} \times a$

But due to friction between $A$ and $B$, body will not move. The body $A$ will start moving when pseudo force is more than friction force.

i.e. for slipping, ${m_A}a = \mu \;{m_A}g$

$\therefore a = \mu \,g$