Question

A block of mass 4 kg is placed on another block B of mass 5kg and the block B rests on smooth horizontal table. If the maximum force that can be applied on A so that both the blocks move together is 12N , the maximum force that can be applied on B for the blocks to move together will be

Answer 1

In the first case, friction acts only between the 4 kg block and the 5 kg block.

So 12 N is the force required for both the blocks to move together,

In the second case, we consider both the blocks as a system and the force acting between them is an internal force.

Thus the ratio of the forces applied on a block to the mass of the block should be a constant.

Now we can write,

$\frac{12}{4}=\frac{F}{9}$

After solving

F = 27 N

Hence the maximum force that can be applied on the block B is 27 N