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-dimentional collision problem.
Press start button to run the animation.
Click the mouse button to pause. Click it again to resume the animation. 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.