A perfectly spherical, but squishable ball of radius \(10~\mbox{cm}\) is blown up to an internal pressure of \(2~\mbox{atm}\). The ball is placed between two perfectly vertical walls and the walls are slowly squeezed together. Initially the ball slips down the walls, but when the walls are \(18~\mbox{cm}\) apart the ball stops slipping down. What is the coefficient of friction between the surface of the ball and the walls?

**Details and assumptions**

- The ambient air temperature is \(20^\circ \mbox{C}\).
- Air has a molar mass of \(29~\mbox{g/mol}\).
- The mass of the ball (not including any air inside) is \(100~\mbox{g}\).
- \(1~\mbox{atm}\) is \(101,325~\mbox{Pa}\).
- The ball squishes as the walls close in, but you may assume the ball does not deform otherwise.
- The acceleration of gravity is \(-9.8~\mbox{m/s}^2\).
- Hint: the answer is very small.

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