*Answer:* Suppose that block exerts a rightward force
on block . By Newton's third law, block exerts an equal and opposite force
on block . Applying Newton's second law of motion to the rightward
acceleration of block , we obtain

where is the mass of block . The normal reaction at the interface between the two blocks is . Hence, the maximum frictional force that block can exert on block is . In order to prevent block from falling, this maximum frictional force (which acts upwards) must exceed the downward acting weight, , of the block. Hence, we require

or

Applying Newton's second law to the rightward acceleration of both blocks (remembering that the equal and opposite forces exerted between the blocks cancel one another out), we obtain

where is the mass of block . It follows that

Since , , and , we have