A child is moving on a skateboard at the speed of 2 ms−1. After

Question

A child is moving on a skateboard at the speed of

2 m s^{- 1}

. After a force acts on him, his speed becomes

3 m s^{- 1}

. Calculate the work done by the force, if the weight of the child is

20 k g

Options:

A . 20 J

B . 50 J

C . 100 J

D . 320 J

Answer: Option B
:
B
Let initial velocity of the child be

v_{1}

and final velocity be

v_{2}

v_{1} = 2 m s^{- 1}

v_{2} = 3 m s^{- 1}

Since the child is in motion, he will have a kinetic energy of

\frac{m v^{2}}{2}

, which will be increasedwhen an external force acts on him.
Work done by the external force on the child

= \frac{m v_{2}^{2}}{2} - \frac{m v_{1}^{2}}{2}

= \frac{20 \times (9 - 4)}{2}

= 10 \times 5 = 50 J

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