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Work Done by Friction

A block is pl...

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 forced 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.

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B

work done against frictions acting on plank and block + elastic energy

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C

elastic energy of string.

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D

difference of work done against friction acting on plank and block.

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Solution

The correct option is A work done against friction acting on plank + energy in the elastic string - work done by friction acting on block.
When plank is moved, the string is under tension. Work done by force $F$ tries to neutralise 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 (a).

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