Question

Three blocks A, B and C having masses 2 kg, 2 kg and 4 kg respectively hang from two ideal pulleys as shown in figure (a). Initially all the three blocks rest on a horizontal floor and the pulleys are held such that the strings just remain taut. At instant t = 0, a force F = 40t newton starts acting on pulley P in upward direction then : $(g=10m/s^2)$

• A. Block A and B lose contact at the same instant.
• B. All the block lose contact at same instant
• C. A lose contact at t = 2 sec.
• D. C never lose the contact

Answer 1

Answer: (A), (B), (C)

The correct options are
A Block A and B lose contact at the same instant.
B All the block lose contact at same instant
C A lose contact at t = 2 sec.

Since the masses A and B are equal, therefore there will be no acceleration. Hence A and B will lose contact at the same instant. Option A is correct.
Further Tension at A= B= 2X10=20N. Tension at C= 4x10=40N.

So C will also lose contact at the same instant as that of A and B. Therefore option B is correct and option D is incorrect.
The total force at the point P due to tension= 40+40= 80N
Therefore,
80=40t
or, t=2sec
Hence option C is correct.