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

(a) v1=20−gt     v2=−20−g(t−2)    v1−v2=[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.