# One thing's for sure. After this, we're all going to be a lot thinner!

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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