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While squeezing a wiffle ball earlier today, I wondered what was causing it to "push back" and oppose the squeezing.

If it was a pressurized ball, I would assume it was the air pressure causing it. If it was a solid ball, I would assume it was something due to the Young's modulus of the material.

However, wiffle balls are neither pressurized nor solid (they are hollow plastic balls with several holes in them). What could be causing this reaction force, and how can it be calculated (assuming more uniform compression, like in between two parallel boards, for instance)?

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  • $\begingroup$ Young's modulus for a solid ball does not make sense. You instead need the bulk modulus of the solid. $\endgroup$
    – Yashas
    Commented Jul 18, 2017 at 9:45
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    $\begingroup$ @Yashas AFAIK Young's Modulus is far more relevant here. The ball is not being uniformly compressed; but is free to deform. This is not a pure volume change of the solid material. $\endgroup$
    – JMac
    Commented Jul 18, 2017 at 15:50
  • $\begingroup$ The changes happen in more than 1 dimension. For small changes, bulk modulus would be more appropriate? $\endgroup$
    – Yashas
    Commented Jul 18, 2017 at 16:03

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The outer shell is stiff and resists deformation.

When you squeeze it you are storing potential energy in the ball because you are making it "out of round". If you let go, the energy is released, returning the ball to its original shape. Just like a spring, but in a different shape.

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  • $\begingroup$ I guess I was more wondering what I could use to calculate the magnitude of this force. Let's say I have a more precise setup in which I am squeezing a ball between two parallel boards. I know that increasing the length of compression (for lack of a better word), but what can be used to calculate that relationship? I will edit my question to better reflect that. $\endgroup$
    – MM1439
    Commented Jul 19, 2017 at 16:32
  • $\begingroup$ I know the calculation for a thin cylinder ($\delta = 0.149 \frac{F r^3}{E I}$) and maybe for a solid one too, but for a thin sphere, I cannot directly find an answer. Have you tried searching for ball compression or stiffness? $\endgroup$
    – John Alexiou
    Commented Jul 19, 2017 at 18:21
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If you cut a whiffle ball in half you get two hemispheres and then into smaller pieces you get curved pieces which have not changed from their original shape.

So a sphere is the equilibrium shape of a waffle ball.
Deforming the waffle ball which is fairly elastic results in forces within the ball resisting the deformations.
Removing the deforming forces results in the ball returning to its original shape not really much different from stretching a spring and then the spring returning to its original length when the stretching forces are removed.

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