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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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Work done against gravity is zero

He does

200 J

work per metre

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

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 \times 9.8 m / s²

= 196 N

(downward direction) and
The displacements along the horizontal direction then, the angle between displacement and force is

θ = 90 °

so, work done,

W = 196 \times s \times \cos 90 °

W = 196 \times s \times 0

(as

s 90 ° = 0

)

W = 0

Hence, work done against the force of gravity is zero.

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