This is a large version of a popular executive toy called Newton’s Cradle. According to various sources, among them Wikipedia and this one, English actor Simon Prebble invented this device in 1967 and named it in honor of Isaac Newton, whose laws it obeys and demonstrates so nicely. It is also known to some as Newton’s Balls. (The writer of one particular web site considers this last name to be less affectionate. I disagree. It is perhaps somewhat indelicate, but who could be unaffectionate towards Newton? . . .but I digress. Also, this last web site and others attribute the invention of this device to Edmé Mariotte, as does a paper in the April, 2012 issue of The Physics Teacher.

This apparatus demonstrates conservation of momentum and energy in an interesting way. When you pull aside a particular number of balls from one end and then release them, they fall back to their original position, and in colliding with the remaining (stationary) balls, transfer their momentum to them such that an equal number of balls emerges from the opposite side with the same velocity the first set of balls had upon collision, and rises to the same height to which the first set of balls was raised. This second set of balls then falls back to its original position, transferring momentum back to the first set of balls. These then fly away from the rest and rise to the height from which they were originally released. The cycle continues until losses from friction, and from the fact that the collisions between balls are not perfectly elastic, damp the motion. (The balls do not quite make it to exactly the same height each time; some of the kinetic energy is converted to heat.) If you release any number of balls, the same number will always fly away from the opposite side, independent of how high you raise them before you release them.

The interesting behavior of this apparatus raises some questions. For instance, for a given starting collision one could imagine a variety of outcomes, all of which would conserve both (kinetic) energy and momentum. (Some of which, admittedly, might be eliminated if one assumed equipartition of energy and momentum among the emerging balls, but would this necessarily be appropriate?) So why does the same number of balls always emerge from the end as the number that struck the opposite end, and with the same velocity as the released balls? Why, for instance, does a single ball not rebound when it strikes the chain of stationary balls? After all, if a single ball struck another ball having five times its mass, it would rebound as it pushed the other ball away. The key is that the energy and momentum are transferred by a series of elastic collisions. The falling single ball, since it has the same mass as the ball it strikes, exchanges all of its momentum with this second ball and is stopped dead. The second ball then exchanges its momentum in similar fashion with the third, and so on, until the next-to-last ball collides with the ball at the opposite end, which, having no other ball to which to transfer its momentum, goes flying off with the same velocity the first ball had when it struck the chain of balls. (See demonstration 24.12 -- Elastic and inelastic collisions on the air track.)

When we examine what happens in the chain of collisions, we see that all balls except for the striking ball and the one at the opposite end experience equal and opposite forces, so that the initial ball is stopped and the end ball flies out while the middle balls remain at rest. When more than one ball is raised and released, the one that strikes the remaining ball(s) experiences the greatest force, and each successive ball behind it a lesser one, with the end ball suffering the least force. The first ball struck by the raised balls suffers the same force as the one that strikes it, and as the collisions progress through the row of balls, the result is that for the end balls the forces are symmetrical to those experienced by the striking balls. Thus, whatever number of balls one raises and releases, the same number flies off the other side. You can find an interesting analysis of this by Dr. Donald Simanek (a retired professor of physics at Lock Haven University of Pennsylvania) here. (The original page appears no longer to be available. This link is to a page on the web archive.)

This demonstration, of course, represents a special case in that all the balls have equal mass. In response to a student's question regarding what would happen if at least one of the balls were heavier than the others, I fashioned a weight that you can slide over the filaments so that it sits atop one of the balls. It would be interesting to make a similar apparatus that had balls of different masses, perhaps also in different patterns. Some such systems are described on the web site mentioned in the last paragraph, and some exhibit the aforementioned rebound of the initial striking ball.

You can also use some clay between two or more balls to illustrate what happens when some of the collisions are inelastic.

You can also try some interesting variations by dropping balls from both sides, both in equal and unequal numbers. As you probably would expect, dropping equal numbers of balls from each side results in equal numbers leaving each side, and dropping unequal numbers of balls results in the same numbers coming out the opposite sides. (That is, for example, drop one ball on the left and two on the right, and get two coming out on the left and one out on the right.)