Freefall Toy

On Einstein's 76'th birthday he was given a toy by a friend. It is reported that he enjoyed showing visitors this toy that demonstrated a key idea of general relativity, the equivalence principal, the equivalence of gravity and acceleration.

Here's how to build a simple version of that toy

Feeefall toy


A cup, any cup will do as long as it is unbreakable when dropped and caught, and large enough in diameter to hold the two balls.

Two rubber balls about an inch in diameter.

Two wood screws

Two rubber bands the rubber bands should be shorter than the height of the cup when relaxed but they should stretch to longer than the height of the cup when loaded with the rubber balls.

Two paper clips.


Attach the rubber bands to the bottom center of the cup. To do this, we poked a hole in the bottom of the cup then tied the rubber bands to a paper clip. We straightened out one paper clip and then put a hook in one end, then used this paper clip to pull the rubber band through the hole in the bottom of the cup.

Attach one rubber ball to each rubber band. Screw a wood screw into each ball then tie the rubber bands to the screws.

To Do and Notice

Hold the cup upright with the balls hanging outside the cup.

Drop the cup.

Notice that as soon as you let go of the cup the rubber bands pull the balls inside the cup.

What's Going On?

When you hold the cup, gravity is pulling down on each ball and each rubber band is stretched and pulling up on its ball with a force equal to gravity.

When you release the cup it goes into freefall and begins to accelerate downward with the acceleration of gravity.

The equivalence principal states that an accelerating frame of reference it is equivalent to a stationary frame of reference with a gravity force opposite the direction of acceleration.

In the freely falling frame of reference of the cup the gravity force due to the downward acceleration is directed upward and is equal and opposite to the force of gravity from the earth. The result is that in this frame of reference the two opposing gravities cancel.

The net force remaining on the ball is that from the stretched rubber band which then contracts pulling the ball into the cup.


Scientific Explorations with Paul Doherty

© 2005

15 April 2005