A man standing on the edge of the terrace of a high rise building throws a stone, vertically up with at speed of `20 m//s`. Two seconds later, an identical stone is thrown vertically downwards with the same speed of `20 m`,. Then

Options

A. the relative velocity between the two stones remain, constant till one hits the ground
B. both will have the same kinetic energy, when they hit the ground
C. the time interval between their hitting the ground it `2s`
D. if the collision on the ground is perfectly elastic, both will rise to the same height above the ground

Answer

A, B, C, D

Explanation

a. `v_(1)=20-"gt"`
`v_(2)=-20-"gt"(t-2)`
`v_(1)-v_(2)=[20-"gt"]+[20+g(t-2)]=20m//s`
Clearly it is time independent i.e., relative velocity between the two stones remains constant.
b. in both the cases, the total energy remains the same.
c. and d. these are simple concepts.