# Mechanics - How to predict the motion after two objects collide in 2D?

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.

One can use law of conservation of momentum to predict the motion, provided that one knows well the momentum of the two particles. So the angle between the motion of the objects after collision can be given by the resultant momentums of the two objects.
But I think as you said, the angle≤1800. Because anyway if we draw two lines in a two dimensional plane, the angle between them cannot be more than 1800.
Centre of mass is a difficult approach. You have to find the centre of mass, see how it moves before collision, and how it moves after collision. Then you have to calculate the angle between the two paths (provided that the objects move along straight lines).

One can use law of conservation of momentum to predict the motion, provided that one knows well the momentum of the two particles. So the angle between the motion of the objects after collision can be given by the resultant momentums of the two objects.
But I think as you said, the angle≤1800. Because anyway if we draw two lines in a two dimensional plane, the angle between them cannot be more than 1800.
Centre of mass is a difficult approach. You have to find the centre of mass, see how it moves before collision, and how it moves after collision. Then you have to calculate the angle between the two paths (provided that the objects move along straight lines).

Erm, can I say that no matter what kind of objects' mass. The angle between two objects will be ≤ 180?
u is the initial velocity of ball in lab frame
v is the initial velocity in center mass reference frame
r is the velocity of the canter mass.
u= v + r
Then the r must be in only one direction?

Yes, although we cannot predict the angle it will always be ≤1800.
And I think 2nd statement is also correct.. r must be in one direction. Because if momentum is conserved, the center of mass velocity of the system is also conserved. Not only that, its velocity must not change i.e, it must be constant.
I just went down a site, I think it might be useful
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