Question

A child is moving on a skateboard at the speed of $2{ms}^{-1}$. After a force acts on him, his speed becomes $3{ms}^{-1}$. Calculate the work done by the force, if the weight of the child is $20kg$.

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

Answer 1

The correct option is B $50J$

Given:
Initial velocity, $v_1=2{ms}^{-1}$
Final velocity, $v_2=3{ms}^{-1}$

Using work energy theorem, work done on the boy is equal to the change in his kinetic energy.
$W=K_2-K_1$
$W=\frac{m{v}_2^2}{2}–\frac{m{v}_1^2}{2}$
$W=\frac{20\times (9–4)}{2}$
$W=50J$