Video: Instructor’s video
Description: This demo dramatically shows the importance of impulse.
Impulse and momentum help explain why the dishes don’t move. If you were to pull the cloth slowly, the dishes would be pulled off the table. The key is that you have to yank the cloth out as quickly as possible. Applying a force for a very short time results in a very small change in momentum, which, if done quickly enough, has almost no visible effect on the dishes.This explanation is somewhat incomplete though; one of the most important factors in this demo is static versus kinetic friction.
When you jerk the cloth suddenly, you actually use a fairly large force to overcome static friction as quickly as possible, then kinetic friction is not only over a short time, but also a smaller force. This minimizes the overall change in momentum of the dishes.
- Tablecloth with at least one smooth edge (no hem).
- Dishes (plate, drinking glass, silverware, candle, and anything else you want to include)
- Put the tablecloth on the table. NOTE: The smooth, unhemmed edge must be placed so that it will pass beneath the dishes when you pull the tablecloth. (otherwise, the hem will catch on the dishes and make them fall).
- Set the dishes on the cloth.
- Often, it increases the drama to set the table in front of the class.
- Take the tablecloth in your hands, with your hands almost as far apart as the table is wide.
- Jerk the cloth as fast as you can and slightly downward, just barely below the plane of the table. (NOTE: if you pull the cloth upward at all, or too far downward, the demo will not work.)
Note: Professors who have not used this demonstration before should practice this before class.