Q. A ball $A$ is thrown up vertically with speed $u$ and at the same instant another ball $B$ is released from a height $h$. At time $t$, the speed of $A$ relative to $B$ is
A) $u$
B) $2u$
C) $u - gt$
D) $\sqrt{(u^{2} - gt)}$
Explanation
(a) At time $t$
[image:3_image211.emf]
Velocity of $A,\ v_{A} = u - gt$ upward
Velocity of $B,\ v_{B} = gt$ downward
It we assume that height $h$ is smaller than or equal to the maximum height reached by $A$, then at every instant $v_{A}$ and $v_{B}$ are in opposite directions
$\therefore V_{AB} = v_{A} + V_{B}$
$= u - gt + gt$ [Speeds in opposite directions get added]
$= u$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.