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Pull The Tablecloth

Students see a classic "magic trick" illustrating inertia. Once you understand the science behind the magic, it will be hard to pull the rug out from under your students!

The glass of water has inertia — it will not move unless a big enough force acts on it. The force pulling on the glass of water is the friction between the tablecloth and the glass. Since the tablecloth and glass are slippery, the friction is quite small — it's enough to pull the glass along slowly, but not enough to accelerate the glass quickly when you snap the cloth fast!

Objectives

  • Describe a scenario that demonstrates the property of inertia.

Materials

  • A tablecloth, cloth (slippery works best), or paper towel with no seams along the edges
    Glass of water (ensure that the outside of the cup is dry)

Key Questions

  • Why does the glass move when the tablecloth is being pulled slowly? Why does the glass stay on the table when the tablecloth is pulled out quickly? Have you ever come inside after a snowy day? Why does kicking your shoes against the wall help the snow come off? What has greater inertia, a glass full of water or an empty one?

What To Do

  1. Using a smooth table top, place the glass of water on top of the tablecloth.
  2. Pulling slowly, move the cup towards the edge of the table. The glass should follow.
  3. With a sudden jerk (don’t hesitate!) pull the cloth from under the glass. One technique is to bunch the cloth up and grab it as close to the table as possible, then yank down and out.The table top and the bottom of the glass need to be smooth and dry so that a minimum of friction exists when the cloth is pulled from under the glass. A silk or satin table cloth works best, and seams will need to be removed.
  4. If you’ve done the trick correctly, you should be able to pull the tablecloth out from under the glass of water with the glass still standing. 

Note: Be sure to practice this before demonstrating it in front of a group, and have some towels ready for clean-up, just in case! An easier demonstration uses wax paper and a karate chop motion to slide the paper out quickly.

Extensions

  • Try the same demonstration with a variety of objects. Can you accomplish the same feat with a whole place setting? Which objects are easier or difficult, and why? Playing Cards and Pennies (another activity in this package) is a way for students to try a similar trick without smashing crockery!

Other Resources

Science World Resources | Full Lesson & other activities | Forces
Science World at TELUS World of Science | School Programs | Roller Coaster