Question

A ball A is thrown vertically upwards with speed u. At the same instant another ball B is released from rest at height h. At time t, the speed of A relative to B is

• A. u
• B. $u - 2gt$
• C. $\sqrt{u^{2} - 2gh}$
• D. $u - gt$

Answer 1

Answer: (A)

u

Answer 2

Taking upwards motions of ball A for time t, its velocity is $v_{A} = u - gt.$

Taking downwards motions of ball B for time t, its velocity is $v_{B} = gt.$

$\therefore$Relative velocity of A w.r.t. B

$= v_{AB} = v_{A} - ( - v_{B}) = (u - gt) - ( - gt) = u$