Math Help - question from newton's law of restitution: e(U1+U2)=(V1+V2)?

1. question from newton's law of restitution: e(U1+U2)=(V1+V2)?

If two balls are travelling to the same direction ,we take it as e(U1-U2)=(V1-V2)
if to the opposite direction we take it as e(U1+U2)=(V1+V2) .my question is ,where should we take this as e(U1-U2)=(V1-V2) if they are travelling to the same direction?(before and after the collision )

2. Originally Posted by silvercats
If two balls are travelling to the same direction ,we take it as e(U1-U2)=(V1-V2)
if to the opposite direction we take it as e(U1+U2)=(V1+V2) .my question is ,where should we take this as e(U1-U2)=(V1-V2) if they are travelling to the same direction?(before and after the collision )
When analysing the collision of 2 objects and you want to determine the final velocity of both objects you need to use the restitution law along with the conservation of momentum.
As regards the direction of travel of each object before and after collision you can assume the sense of the unknown velocity. If your solution yields a negative magnitude, then the velocity acts in the opposite to what you assumed.