Posted by **your hero....** on Sunday, April 28, 2013 at 4:35am.

A perfectly spherical, but squishable ball of radius 10 cm is blown up to an internal pressure of 2 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 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∘C.

-Air has a molar mass of 29 g/mol.

-The mass of the ball (not including any air inside) is 100 g.

-1 atm is 101,325 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 m/s2.

Hint: the answer is very small.

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