Question

A block is placed on a plank which is placed on a horizontal plane. A massless but elastic string deviates by an angle θ with vertical when a force F is applied to the plank to shift it to the right making the block slide over it. If F_P and F_B are frictional forces between plank and plane and between block and plane respectively then work done by applied force is given by

• A. Work done against friction acting on plank + energy in the elastic string - work done by friction acting on block.
• B. Work done against frictions acting on plank and block + elastic energy.
• C. Elastic energy of string.
• D. Difference of work done against friction acting on plank and block.

Answer 1

Answer: (A)

Work done against friction acting on plank + energy in the elastic string - work done by friction acting on block.

Answer 2

When plank is moved, the string is under tension. Work done by force F tries to neutralize the frictional force of plank and elongation of the string due to non sliding of the block due to its friction. Thus, work done by F is equivalent to option (1).