Q. A body of mass 5 kg starts from the origin with an initial velocity $\mathbf{u =}\left( 30\widehat{\dot{\mathbf{i}}} + 40\widehat{\dot{\mathbf{j}}} \right){\ ms}^{- 1}$. If a constant force $\left( - 6\widehat{\dot{\mathbf{i}}} - 5\widehat{\dot{\mathbf{j}}} \right)$N acts on the body, the time in which the $y$ component of the velocity becomes zero is
A) 5 s
B) 20 s
C) 40 s
D) 80 s
Explanation
(c) From Newton’s second law of motion,
$Force = mass \times acceleration$
Acceleration along $y$-axis is
$a_{y} = - \frac{5}{5}{ms}^{- 2} = - 1{ms}^{- 2}$
Form equation of motion
$v_{y} = u_{y} + a_{y}t$
Where $v_{y}$ is initial velocity along $y$-axis, $t$ the time and $v_{y}$ the final velocity along $\ \ \ \ y$-axis.
$Given,\ \ v_{y} = 0,\ u_{y} = 40\ {ms}^{- 1},\ \ a_{y} = - 1\ {ms}^{- 1}$
$\therefore\ \ 0 = 40 - 1 \times t$
$\Longrightarrow t = 40\ s$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.