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Playing Cards and Pennies

Challenge your students to put a penny in a cup without touching it.

The solution is to use inertia to your advantage by taking the playing card out of the way as quickly as possible.

This is a simple (and less costly!) way to practice the "waiter's trick" of pulling a tablecloth out from under a plate. This is a demonstration they can easily recreate at home without breaking all of the dishes!

A full explanation of the physics involved is included in the Pull the Tablecloth resource.

Objectives

  • Describe a scenario that demonstrates the property of inertia.

Materials

  • Per student or small group:
    a cup
    a penny
    playing or index card (stiff paper)

Key Questions

  • Before the demonstration is performed, what forces are acting on the cup?
  • Before the demonstration is performed, what forces are acting on the card?
  • When you flick the card, what forces are acting on the penny?
  • Does this trick work with a lighter object (like a cotton ball)? Why or why not? Experiment and see how light an object can be for the trick to work.

What To Do

  1. Have students set up a playing card over the mouth of a cup, and place a penny on top, directly over the centre of the cup.
  2. Challenge them to figure out how to get the penny in the cup without touching the penny or lifting the card.
  3. Once they have found the solution, have them try with a stack of pennies. How many pennies can they drop into the cup at once?

  • Solution: Students should flick the card swiftly with their finger, to send it out from under the penny, allowing it to drop into the cup.