My coworker makes rubber band balls by wrapping rubber bands one over another. At the initial stage it was pretty bouncy. Now, as it has become larger and denser, it seems to have lost its bounciness. Could anyone give me a simple physics based explanation? I have a hunch it has to do with both increasing weight as well as the decreased spacing between adjoining rubberbands.
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$\begingroup$ Are you using rubber bands of the same size at all layers? I suspect that the amount by which a rubber band is stretched out of its equilibrium length affects the bounce, since I know that at some point stretched rubber bands stop being rubbery and snap. $\endgroup$– g sCommented Jan 19 at 9:03
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$\begingroup$ I was thinking in that track myself, but could not verbalize at first. Maybe I needed and ally like you. You are right about the suspicion, since my coworker was using the bands from the same package and we can assume he was using bands of the same average thickness and diameter. Based on that, the bigger the ball becomes, the stress on the bands will be progressively higher on the outer layers of the ball than the inner one. And as you say, its being rubbery will be somehow inversely related to the radius of the sphere, given the constant average composition and ddimension of each band. $\endgroup$– Kafi KfishnaCommented Jan 21 at 8:55
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2 Answers
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the most likely explanation is that as the ball gets bigger, internal friction (adjacent bands rubbing against one another) gets bigger because the successive layers of bands are not well-bonded to each other.
This means that the bigger the rubber band ball gets, the more impact energy gets dissipated when the ball strikes the ground.
This can be easily tested by comparing the bounce results of a rubber band ball against that of a solid rubber ball of similar size.
answered Jan 19 at 4:59
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$\begingroup$ This might be true, but I don't think the proposed test isolates the variable. The stretch-ed-ness of the rubber bands seems significant here, and you can't buy a rubber ball with the rubber of the outer layer pre-stretched. $\endgroup$– g sCommented Jan 19 at 9:00
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$\begingroup$ @nielsen: Thank you. Now is it fair to assume that after a critical point, given each band is of same average spec in mass and dimension, the density of the rubber band sphere goes down as it gets bigger? Since a solid rubber ball of the same dimension will have higher density, due to less air gaps/pockets than a same size sphere made of rubber bands? $\endgroup$ Commented Jan 21 at 9:06
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$\begingroup$ @KafiKfishna, I do not know! but an experiment would tell us readily! $\endgroup$ Commented Jan 21 at 19:26
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Simple and short explanation is that the bigger the ball, the more difficult it is to efficiently occupy the bulk space with rubber bands. As you keep adding more rubberbands, progressively higher percentage of the ball's volume gets occupied by air pockets instead of rubber. The presence of such pockets dampens the elastic deformation that is necessary for the bounciness; the act like a damper in car's suspension.
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$\begingroup$ "As you keep adding more rubberbands, progressively higher percentage of the ball's volume gets occupied by air pockets instead of rubber." Citation needed. This would suggest that at some point, adding another rubber band encloses more air than adds rubber. This doesn't seem credible, since the increased stretching with increasing volume seems more likely to compress the interior than to "aerate" it. $\endgroup$ Commented Jan 18 at 21:29