The figure shows a 20 kg block resting on a frictionless surface and being pulled by a force F. On it rests a 5 kg block with the coefficient of friction between the 20 kg block and the 5 kg block being 0.5.
The force of the friction between the 5 kg block and the 20 kg block is F = `mu*N` = 0.5*5*10 = 25 N
If the lower block of 20 kg is pulled with a force equal to 25 N, the block placed above it does not slip.
The maximum force with which the 20 kg block can be pulled by the string such that the 5 kg block does not slip is 25 N
The correct answer is option A.