Teaching Physics with the Physics Suite

Edward F. Redish

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Why can't I pick the &*@#$ thing up?

On Professor Redish’s desk, he has a pencil holder made of a piece of bamboo. Around the bottom of the bamboo is a soft circular disk of soft rubber. This is shown in the figure at the right.

A. If you grasp the bamboo tube and try to lift it, you will find it extremely difficult as a result of the air pressure holding down the disk. Given that atmospheric pressure is about 100 kP (= 105 N/m2), estimate the upward force you would need to exert in order to lift the tube. Be sure to clearly state your assumptions and how you came to the numbers you estimated.



B. However, after you have tried hard to lift the tube (and possibly succeeded but with great effort) Professor Redish lifts the tube easily – by sneakily lifting the corner of the disk in the back with his little finger as shown in the figure at the right. Explain why this makes it much easier to lift the bamboo tube.

Page last modified December 28, 2010: M34