Perform a Law of Motion Magic Trick

What You Need:

  • Sheet or tablecloth
  • Table
  • Heavy, non-breakable object (textbooks work great)

What You Do:

  1. Have your child place a sheet on the table. Make sure the sheet or tablecloth has a smooth edge, a ridge along the side will make it more difficult to pull it smoothly from under the book.
  2. Place the book on the cloth. Initially rest the book about 1-2 feet from the edge of the tablecloth. As your little Houdini becomes more adept at the trick, she can gradually work up to centering the book on the cloth.
  3. Have your child quickly pull the cloth out from under the book. Be sure to have her pull the cloth quickly (think of a snapping motion) and slightly down on the cloth (if she doesn’t understand why, have her try the trick while pulling slightly upward and see what happens). Can she pull the cloth out from under the book with minimal disturbance to the book?
  4. Practice makes perfect! This is a trick that requires a little skill and a little practice. As she gets better at the trick your child can opt to place more impressive objects on the cloth like dishes, but always think twice about using the fine china!

What's Going On?

As with everything, there are several science concepts at work, but the key concept is Newton’s First Law of Motion. Ask your aspiring magician to recite Newton’s First Law. She should say something similar to:

An object at rest remains at rest and an object in motion remains in motion, in a straight line, unless it is acted upon by an outside force. This is also known as the concept of “inertia.”

Have her explain how this law applies to the trick. The book is at rest: due to its inertia it will stay at rest unless something makes it move. By pulling the cloth (applying an outside force) we expect the book to move and in fact, it will move if the cloth isn’t pulled quickly enough. There is friction between the book and the cloth, so if the cloth is not pulled quickly the book will move with the cloth. By applying enough momentum to the cloth, the friction between the two objects is overcome and the book remains at rest. Because of friction, the book will move slightly before it comes to a complete rest again (can you spot this motion?) and the better the trick is performed, the less noticeable the motion becomes. Presto!

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