Question

A body of mass $5\, kg$ is moving with a momentum of $10\,kg\,m/s$ . A force of $0.2\,N$ acts on it in the direction of motion of the body for $10\, s$. The increase in its kinetic energy is

• A. $2.8\,\,J$
• B. $3.2\,\,J$
• C. $3.8\,\,J$
• D. $4.4\,\,J$

Answer 1

Answer: (D)

$4.4\,\,J$

Answer 2

Change in momentum $=$ force $\times$ time $\Rightarrow p_{2}-p_{1}=0.2 \times 10=2\, kg\, m / s$ $\Rightarrow p_{2}=p_{1}+2=10+2=12\, kg\, m / s$ Increase in $KE =\frac{1}{2 m}\left(p_{2}^{2}-p_{1}^{2}\right)$ $=\frac{1}{2 \times 5}\left[(12)^{2}-(10)^{2}\right]$ $=\frac{1}{10} \times(144-100)$ $=4.4\, J$