Q.

A block of mass m is directly pulled up slowly on a smooth inclined plane of height h and inclination θ with the help of a string parallel to the incline. Among the following statements, the one that is incorrect for the block when it moves up from the bottom to the top of the incline is

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a

Work done by gravity is mgh

b

Work done by the string is mgh

c

Work done by the normal reaction force is zero

d

Net work done on the block is zero

answer is C.

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Detailed Solution

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Block is slowly moved it means net acceleration is zero, so

T=mgsinθ

(i) WN = 0, because normal reaction is perpendicular to displacement.

(ii) WT=mgsinθ×hsinθ×cos0= mgh

(iii) Wg = mg x component of displacement along mg

So Wg=mg(h)=mgh

(iv) By work energy theorem

WAll =ΔKE

But, ΔKE=0, So WAll=0

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