PURPOSE: To show the equivalence principle.
DESCRIPTION: A body in freefall experiences no gravitational force. Otherwise stated, one cannot possibley distinguish the difference between a gravitational force and an acceleration. This demo was derived from a modification of an old fashioned toy for children.
A long broomstick has a cup attached to the end. Inside the cup is a heavy ball and a weak spring. When the ball is made to hang out of the cup, as shown in the photo above left, the spring is too weak to pull it back in. But when the stick is raised up in the air and released, as shown in the photo above right, gravity is instantly switched off. In the absence of gravity, while in freefall, the weak spring has the ability to pull the ball back into the cup.
There are two versions of this apparatus. One is a stick with a ball and a spring. Another version has a stick with a cup that has two weak magnets seperated by a small gap. When in freefall the magnets can overcome gravity to become instantly attracted together.
There is a good article about the history of this demo from the Scientific American, July 1955. This apparatus was designed by Eric Rogers of Princeton and presented to Einstein on his 76th birthday.
EQUIPMENT: Equivalence principle apparatus.
Updated by Jun Qi in 3/20/2000