Q. A ball $A$ is thrown up vertically with a 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
Solution:
From equation of motion,
we have $ v=u+gt $
where u is initial velocity and t is time.
For ball A, $ {{v}_{A}}=u-gt $
For ball B, $ {{v}_{B}}=0+gt $
$ \therefore $ Relative speed, $ \Delta \overrightarrow{v}={{\overrightarrow{v}}_{A}}-{{\overrightarrow{v}}_{B}} $
$ =(u-gt)\hat{j}-(-gt\,\hat{j}) $
$ =u\hat{j} $
