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Question

# A body is displaced from position A to position B. Kinetic and potential energies of the body at positions A and B are KA=70 J, UA=−50 J ,KB=−20 J and UB=30 J. Then:

A
Work done by conservative forces is 80 J.
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B
Work done by all the forces is 90 J.
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C
Work done by non-conservative forces is 10 J.
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D
Work done by all the forces is 80 J.
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Solution

## The correct options are A Work done by conservative forces is −80 J. B Work done by all the forces is −90 J. C Work done by non-conservative forces is −10 J.Work done due to conservative force =− (Change in P.E.) ⇒Wc=−ΔU ⇒Wc=−(UB−UA) ⇒Wc=−(30−(−50) ⇒Wc=−80 J Work done by all forces = change in K.E. ⇒Wall=KB−KA ⇒Wall=−20−70 ⇒Wall=−90 J Let Wnc be the work done by the other (non-conservative) forces. Wall=−90 J ⇒Wc+Wnc=−90 ⇒−80+Wnc=−90 ⇒Wnc=−90+80 ⇒Wnc=−10 J Hence, options (a), (b) and (c) are correct.

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