Drive a wooden skewer through one of the styrofoam balls. Trim skewer so that about 3/4 of an inch of the skewer sticks out from each end.
Tie one end of the piece of string around the skewer. You can optionally reinforce this with a piece of tape or hot glue.
Poke a T pin through the foam or cork, with the head of the pin on the bottom of the foam or cork.
Stick the prongs of three forks into your piece of foam or cork, spacing them evenly apart to form a tripod.
Place the tripod on a large dinner plate.
Tie the other end of the twine that is attached to the other foam ball around the head of the pin. You will want to make sure that the wooden skewer of the apple is nearly touching the plate but can still move freely. Trim your string as necessary.
Gently tap or push the bottom foam ball to begin swinging like a pendulum. At the two opposite ends of the swing on the plate, make a pile of about 1 teaspoon of salt on each end. You will want the skewer that is sticking out of the foam ball to go through each pile of salt, so add more salt if necessary to achieve the right height.
As the pendulum swings through the salt, you'll notice it starts to create little grooves in the salt. Now, slowly and gently turn the plate and you will see new grooves forming, but the direction of the pendulum does not change.
Notes
This experiment was originally published in the 1926 Groiler's Book of Knowledge Encyclopedia for Children, which is now in the public domain. We've adapted it a bit to be easier to recreate at home or in the classroom.The original activity suggested using an apple and cork, but foam balls are usually a little easier to work with. Styrofoam wasn't invented until 1940 though and this experiment is from 1926!This model can be adapted to be made from almost nearly any material, for example, you could use K'Nex building pieces or you could use wooden dowels or large craft sticks instead of forks. You could also choose to use sand instead of table salt.Using a dark solid colored dinner plate or a white plate with colored sand will make it easier to see the lines drawn from the pendulum as it swings.