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by Lori Stewart
Topics: Middle School, Science
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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!