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Question

# A child is moving on a skateboard at the speed of 2 ms−1. After a force acts on him, his speed becomes 3 ms−1. The work done by the external force on the child is ___J. Given, the weight of the child is 20 kg.

A

100 J

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B
50 J
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C
20 J
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D
320 J
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Solution

## The correct option is B 50 JLet initial velocity of the child be v1 and final velocity be v2 v1=2 ms−1 v2=3 ms−1 Since the child is in motion, he will have a kinetic energy of mv22, which will be increased after an application of force by an external agent on him. Work done by the external force on the child =mv222–mv212 =20×(9–4)2 =10×5=50 J

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