Bernoulli and collapsing paper tents

This is the simplest Bernoulli demonstration I have seen though there is some subtlety to the explanation. It is also great because it is easy to get 100% class participation. I believe that the demonstration has been around for a while but thanks to Jean Hertzberg for telling me about it.

Equipment

  1. A sheet of paper

tent

Demonstration

  1. Fold the paper in half along the long axis of the sheet to form a tent
  2. Place the tent on a table and blow into one end.
  3. The tent should collapse down starting at the end you blow through

Discussion

When you blow into the end of the tent the air jet from your mouth induces a flow of ambient air into the tent. Applying Bernoulli’s equation along a streamline from the stationary ambient into the tent (see the figure below) leads to

0=U22/2g + P2

Therefore, P2 must be negative (sub-atmospheric). Therefore, there is a pressure imbalance across the tent walls near the entrance and the tent collapses.

Presentation1

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

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