• table with flat edge
1. Lay the ruler over the edge of the table so that ~1/3 of it's length is over the edge.
2. Ask your audience what will happen if you hit the ruler from above.
3. Hit the ruler – as expected it flips off the table.
4. Ask your audience how you might possibly keep the ruler on the table while you hit it, using only newspaper. Hopefully someone will guess that you need to exert an opposing force on the far end of the ruler – you may need to prompt them.
5. Tell your audience that you can only use a sheet of newspaper. Try first by folding up a sheet of newspaper as small as possible and placing it at the back end of the ruler so that it acts as a counterweight. Get an audience member to hit the ruler again – still it flips off the table, this time along with the folded up newspaper!
6. Ask your audience how else you might be able to use a sheet of newspaper to hold the ruler down. If your audience guess the trick, ask them to explain the physics behind the idea. Lay a single sheet of newspaper flat on the table so that the ruler is roughly in the center. When you hit the ruler it will stay on the table!
How does it work?
It all comes down to air pressure. Atmospheric pressure is exerting a downward force on the single sheet of newspaper. The area of a single sheet of newspaper is fairly large, therefore the downward force of the atmospheric pressure exerted on the newspaper is strong enough to counter the upward force of hitting the ruler. It didn't work with the folded-up newspaper because the surface area over which the atmospheric pressure could act was far too small.
Tips for Success
For optimal effect, make sure as little air as possible is under the newspaper by smoothing it out flat prior to hitting the ruler.
This works well with audiences of all ages, but does require a sturdy table or bench, so is best suited for performing when an audience has already been gathered.
Did You Know?
During the scientific revolution it was common to think of air pressure in terms of the total weight of a column of air pressing down on a unit area. In 1643 Evangelista Torricelli, a pupil of Galileo, inverted a mercury filled glass tube, sealed at one end, into a basin also containing mercury. He found that the weight of air over the basin was sufficient to support a column of mercury to a height of 76 cm. This invention is the basis of using ‘millimetres of mercury’ as a unit of air pressure.