A body of mass `2 kg` is projected upward from the surface of the ground at `t = 0` with a velocity of `20 m//s`. One second later a body B, also of mass `2 kg`, is dropped from a height of `20m`. If they collide elastically, then velocities just after collision are :-

Options

A. `v_(A) = 5 m//s` downward, `v_(B) = 5 m//s` upward
B. `v_(A) = 10 m//s` downard, `v_(B) = 5 m//s` upward
C. `v_(A) = 10 m//s` upward, `v_(B) = 10 m//s` downward
D. both move downward with velocity `5 m//s`

Answer

A

Explanation

After `1 s , v_(A) = 20 - 10 xx 1 = 10 m//s`
and `v_(B) = 0 + 10 xx 1 = 10 m//s`
At the time of collision
`V_(A) = V_(B) = 5m//s`
after collision, velocity gets enterchanged.