Vacuum needed for coin-feather tube?

  • Thread starter mishima
  • Start date
  • #1
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Main Question or Discussion Point

There is a common free fall demonstration that involves a long evacuated cylinder, a coin, and a feather. The tube is plexiglass and so the fall of the coin and feather can easily be seen as synchronized.

I'd like to put one of these together, but was curious about the vacuum needed for a good demonstration. I would prefer not buying an expensive vacuum pump (just yet).

There is a small aquarium pump at wal-mart for something like 7 dollars, would this pull enough? There are other cheap pumps for camping purposes. Another option is MityVac, a hand operated pump used for automotive repair, which goes for maybe $50.

Has anyone accomplished this without a "real" pump? What is the maximum atmosphere inside the cylinder so they both hit at the same time?
 

Answers and Replies

  • #2
898
67
Drag is proportional to density, so you don't need a good vacuum. Even at 1 mbar you have already reduced the drag by a factor of 1000 compared to normal atmospheric pressure.

A cheap electric car tire inflator should do the job. Just make sure you find one where you have access to the air inlet.
 

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