A ball is dropped from the building and simultaneously another ball is projected upward with some velocity. Describe the change in relative velocities of the balls as a function of time.

Explanation

a) For a body dropped from building its velocity `v_1=g t to (1) ( because u_1=0)`
b) For a body thrown up with a velocity `u` its velocity , `v_2=u- g t to (2)`
`because` The two balls are moving in opposite direction the relative velocity.
`v_R=v_1+v_2`
`thereforev_R=g t+u- g t =u`
Here the relative velocity remains constant, but velocity of one body increases at a rate of `g` `m//sec` and velocity of another body decreases at a rate of `g` `m//sec`.