Q. A block of mass $5kg$ is resting on a smooth surface. At what angle a force of $20N$ be acted on the body so that it will acquired a kinetic energy of $40J$ after moving $4m$
A) $30{^\circ}$
B) $45{^\circ}$
C) $60{^\circ}$
D) $120{^\circ}$
Explanation
(c) According to work-energy theorem
$W =$ Change in kinetic energy
$FS\cos\theta = \frac{1}{2}mv^{2} - \frac{1}{2}mu^{2}$
Substituting the given values, we get
$20 \times 4 \times \cos\theta = 40 - 0$ $\lbrack\because u = 0\rbrack$
$\cos\theta = \frac{40}{80} = \frac{1}{2}$
$\theta = \cos^{- 1}\left( \frac{1}{2} \right) = 60{^\circ}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.