Newton's First Law of Motion: Inertia—the Magician's Friend

By — John Wiley & Sons, Inc.
Updated on Dec 13, 2010

Sir Isaac Newton is well known for his three laws of motion. The first law of motion, the law of inertia, states that objects at rest will remain at rest, and objects in motion will remain in motion, unless acted on by an outside force. Inertia, a resistance to change, depends on both mass and friction. In this activity you will observe the inertia of a penny at rest.


Plastic cup; Index card; Penny


  1. Place an index card on top of the mouth of the plastic cup.
  2. Put the penny on top of the card, so that it sits directly above the mouth of the cup.
  3. Using your finger, thump the index card horizontally so the card flies off the cup and the penny drops into the cup below.
  4. Repeat this process until you have mastered this activity.

Newton's First Law of Motion

Follow-Up Questions

  1. Explain how this activity demonstrates the law of inertia.
  2. How is this activity similar to the magician's trick of jerking the tablecloth from under a place setting of dishes without breaking any dishes?
  3. Do you think the texture of the tablecloth is important to the magician?


  1. he penny was at rest, and when you thumped the card from under it quickly, the penny remained at rest. It simply fell into the cup, rather than flying away with the card.
  2. Both demonstrated the law of inertia.
  3. The rougher the texture of the cloth, the less likely the trick will work. The friction of the tablecloth could place a horizontal force on the dishes, causing them to move with the cloth rather than stay put on the table.


Try this activity with a quarter rather than a penny or with sandpaper rather than an index card.

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