I think no matter what are the mass of two objects, there must be a fix angle between the angle of two direction. Is that using the center mass reference to calculate? I say that because we can prove that when the two objects with same mass collide, then the angle between the direction of the objects'motion will be 90°. That is proven based on the condition that they have same mass, what if different mass, do we have any fixed angle (eg the angle ≤ 180°) These are elastic collision. please guide . thank you.