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Question

A child is moving on a skateboard at the speed of 2 ms1. After a force acts on him, his speed becomes 3 ms1. 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 J

Let initial velocity of the child be v1 and final velocity be v2
v1=2 ms1
v2=3 ms1
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 =mv222mv212

=20×(94)2

=10×5=50 J


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