Friction..

Two trolleys $1$ and $2$ are moving with accelerations $a_1$ and $a_2$ respectively in the same direction. A block of mass $m$ on trolley $1$ is in equilibrium from the frame of observer stationary with respect to trolley $2$ . Find the magnitude of friction on the block due to the trolley.

Assume: No horizontal force other than friction is acting on the block.