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Q.

A man holds and displaces a 20 kg mass horizontally in his stretched hand. Then which of following statements is/are true?


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a

Work done against gravity is zero

b

He does 200 J work per metre

c

The work done against gravity cannot be estimated but is non-zero

d

The work done against gravity is equal to the muscular energy used 

answer is A.

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Detailed Solution

The man does zero work against the force of gravity.
Work done is the dot product of force and displacement.
so, W = F.s.cosθ
According to the question, A man holds and displaces a 20 kg mass horizontally in his hand.
so, the gravitational force acting on the body,
 F = mg
      = 20 kg × 9.8 m/
      = 196 N (downward direction) and
The displacements along the horizontal direction then, the angle between displacement and force is θ = 90°
so, work done,
= 196 × s ×cos 90°
W= 196×s×0        (as 90°=0 )
W= 0
Hence, work done against the force of gravity is zero.
 

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