Phantastic Physics Demos: The Card Blow

A great proof of Bernoulli's principle is to fold an index card so that about a cm on both ends has been folded at a right angle to the rest of the card. Next, place the card on a flat surface with the folded parts facing down. Now challenge someone to blow the card off the table by blowing under the card. The harder the person blows under the card the more the card sits there. It even bends down closer to the surface as air passes underneath the card. Once again the experience gets explained in terms of Bernoulli's principle.

When the faster moving air (shown in the video as blue lines) passes under the card, this faster moving air applies less force on the underside of the surface. The slower moving air above the card is then allowed to push the card down toward the surface you set the card on.

This demonstration can also be done with an index card folded in half, like a tent or A-frame.