Question

Two identical spheres move in opposite directions with speeds v₁ and v₂ and pass behind an opaque screen, where they may either cross without touching (Event 1) or make an elastic head-on collision (Event 2).

(a) We can never make out which event has occurred. 

(b) We cannot make out which event has occurred only if v₁ = v₂. 

(c) We can always make out which event has occurred. 

(d) We can make out which event has occurred only if v₁ = v₂. 

Answer 1

Correct answer is: (a) We can never make out which event has occurred.

If a collision does occur, the velocities of the two spheres will interchange. If a collision does not occur, they retain their original velocities. As the spheres are identical, an observer cannot tell from the velocities of the two spheres whether a collision occurred or not.