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Published: 24 June 2021

• energy
• mechanics
• simulation

This little toy, C7-18 in our collection and sold in many shops as the Astro Blaster, is a fun way to demonstrate some interesting and complicated collision physics. John Ball presents it in this video:

 This device has four balls of graduated masses on a central shaft. When the whole assembly is dropped, the smallest ball on the end flies with considerable velocity, potentially rising to significantly greater than the initial height.

 The balls are highly elastic, and when they collide, they transfer much of their energy to the smallest ball, which has a slightly larger hole in it and thus is the only one free to move off the shaft. Since it now carries the kinetic energy of the greater mass of falling balls, it can bounce higher than it started! Meanwhile, the rest of the balls fall quietly (more or less) to the surface.

It is believed that the use of this kind of collision in physics classes was initiated by Stirling Colgate of Lawrence Livermore National Laboratory. It was also popularized by an article in The Physics Teacher by Richard Mancuso and Kevin Long,  https://aapt.scitation.org/doi/abs/10.1119/1.2344238 .

 Once you’ve seen it in action in the video, you can also try it out in this simulation. Or try it at home with a couple of elastic balls, one high mass and one low mass, such as a small rubber ball and a well inflated basketball. If you can drop them together (this takes practice), you should see the smaller ball bounce away with much greater velocity. Just try not to break anything!