Stopping a Bounce

One way to damp a bouncing ball is to partially fill it with a fluid (a) or granular material (b). For the fluid, the initial impact sloshes the liquid. That doesn’t change the trajectory of the initial bounce noticeably, but it interferes with the second impact, drastically damping the rest of the ball’s bounces until it comes to a stop. A grain-filled ball is similar, at least to begin with. The initial bounce sends the grains flying, forming a granular gas inside the ball. This doesn’t affect the trajectory of the first bounce, but the second impact collapses the granular gas. All the impacts of the grains with one another dissipate the energy of the bounce, and the ball comes to a complete stop. This suggests that a partially-grain-filled container can make a good damper in sport or industrial applications. It also suggests that it might be even better for water-bottle flipping than water is. (Image and research credit: F. Pacheco-Vázquez & S. Dorbolo)

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