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 $m{s}^{-1}$. Find the work done by the external force on the child, given the mass of the child is 20 kg.

• A. 100 J
• B. 50 J
• C. 20 J
• D. 320 J

Answer 1

The correct option is B 50 J

Let initial velocity of the child be $v_1$ and final velocity be $v_2$
Given, $v_1=2m{s}^{-1}$, $v_2=3m{s}^{-1}$ and mass of the child $=20kg$.
Since the child is in motion, he will have a kinetic energy of $\frac{m{v}^2}{2}$, which will be increased after an application of force by an external agent on him.
Now, Applying work energy theorem ,
Work done by the external force on the child $=\frac{m{v}_2^2}{2}–\frac{m{v}_1^2}{2}$

$=\frac{20\times (9–4)}{2}$

$=10\times 5=50J$