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Pull The Tablecloth

In this activity, students see a classic "magic trick" illustrating inertia.

Once they understand the science behind the magic, it will be harder 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 in this activity 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

  • Per Demo or Group:
    Table or smooth, level surface
    Tablecloth, cloth (slippery works best, like silk or satin, and seams will need to be removed), 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?
  • What has greater inertia, a glass full of water or an empty one?
  • Have you ever come inside after a snowy day and cleaned off your boots? Why does kicking your shoes against the wall help the snow come off?

What To Do

  1. Using a smooth table top, place the glass of water on top of the dry tablecloth or paper towel.
  2. Pulling slowly, move the cloth 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!

OPTIONAL: 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 more difficult to use, and why?
  • Can you accomplish the same feat with a whole place setting?
  • Teacher Note: Playing Cards and Pennies (another activity in this package) is a way for students to try a similar trick without smashing crockery!