Riki tiki

Rhythmical movement of your hand supplies two hard balls with the amount of energy equal to the energy lost in the collision, therefore the balls neither accelerate nor slow down.

On the other hand, after watching closely the way in which the balls are made to collide you can come to the conclusion that the direction of the hand movement and the directions of the centres of gravitation movement of the balls must be aligned correctly. That means that collisions must be central, if you want the balls collide regularly.

Riki - Tiki is a kind of Newton's Pendulum with two hard balls at the lower end. Another difference is that both balls can swing. Let us denote the initial velocity of the balls with v and -v their velocity after the collision with V₁ i V₂.

The conservation of momentum equation has the following form:

mv - mv = mV₁ + mV₂ this in turn leads to the following conclusion: V₁ = - V₂

and the energy conservations says:

½mv² + ½mv² = ½mV₁² + ½mV₂²

Having taken the first equation into consideration, the following result is obtained |V₁| = |V₂| = |v|. In the above result, only absolute velocities are obtained. There is no indication which of the V₁ or V₂ is positive and which is negative. As a matter of fact, we do not know which of the two balls bounced off to the right and which bounced off to the left (and even whether or not the collision came about...).