The Newton's Pendulum

The Newton's pendulum is example of collision between two bodies of the same mass - like two billiard-balls. After the collision, the two bodies fly away at a right angle.

But if the collision is perfectly central, as it should be for the pendulum, the first sphere stops and the other moves away. The two spheres exchange their quantity of motion (momentum) and energy.

The games becomes unpredictable if we take one sphere away. Collisions aren't perfectly central anymore and the motion becomes chaotic soon.