Question

A block of mass $5\, kg$ is resting on a smooth surface. At what angle a force of $20\, N$ be acted on the body so that it will acquired a kinetic energy of $40\, J$ after moving $4\, m$ ?

• A. $30^{\circ}$
• B. $45^{\circ}$
• C. $60^{\circ}$
• D. $120^{\circ}$

Answer 1

Answer: (C)

According to work-energy theorem $W=$ Change in kinetic energy
$F S \cos \theta=\frac{1}{2} m v^{2}-\frac{1}{2} m u^{2}$
Substituting the given values, we get
$20 \times 4 \times \cos \theta=40-0 \,\,(\because u=0)$
or $ \cos \theta=\frac{40}{80}=\frac{1}{2}$ or
$\theta=\cos ^{-1}\left(\frac{1}{2}\right)=60^{\circ}$