<!param name="m1" value="m1"><!param name="m2" value="m2"><!param name="eta" value="eta"><!param name="frame" value="frame">
Newton's law of motion look the same to all observers in inertial frames of reference.
It is equally true that if momentum is conserved in one inertial reference frame, it is conserved in all inertial frames.

This java applet apply the above concept to one dimensional collision problem.
Two circular objects are confined to move in one diminution (between two gray blocks).
Press start button to run the animation.

Circular objects will move with some pre-defined velocity (yellow arrow).

Click the mouse button to pause. Click it again to resume the animation.

While the animation is suspended:

Click near the arrow of the velocity vector and drag it left/right to change the initial velocity.
Click at the center of the circle to move it : drag the object left <--> right.
Click within the circle to change the mass of that object.

Click right mouse button to increase mass by one unit.
Click left mouse button to decrease mass one unit.

Press Reset to reset most parameters to default values.

eta is the coefficient of restitution
  eta = | relative velocity just after collision/relative velocity just before collision |
for elastic collision eta=1., for perfectly inelastic collision eta=0.

You can select different frame of reference to view the relative motion of all the objects.
  lab is a laboratory inertial frame.
  m₁, m₂ and CM are frame of reference with respect to left circular object m1, right circular object m2 and center of mass for m1 and m2.
View collision in 2 dimension.