The Demo: The beaker stays put when the tablecloth is pulled out from under it. Why?
Quick Physics: Newton’s First Law of Motion states that an object at rest will stay at rest unless acted on by an unbalanced force.
The Details: This “magic trick” very nicely demonstrates the Law of Inertia. Inertia is a measure of how hard it is to change an object’s motion. The heavier an object is, the more inertia it has – it’s harder to make a heavy object move! This means that if something it sitting still, its inertia keeps it in place.
In the demonstration a big beaker of water is filled with water. It is put on a tablecloth made of some slippery material like rayon or polyester. If the tablecloth is pulled slowly, friction (rubbing) between the beaker and tablecloth will cause the tablecloth to pull the beaker with it. But, if the tablecloth is pulled quickly, the beaker won’t mover. Inertia keeps the beaker in place because friction is not as strong when you pull fast. The trick works no matter how heavy an object is since friction is weaker for lighter objects. Provided you pull quickly enough, this trick can even be done with glasses or plates.