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Teaching Space: Free Fall

By Jim Plaxco

So you're sitting around at home watching CNN with your kids and on comes a story about the current space shuttle mission. Your son or daughter sees the astronauts floating around and turns to you and asks "How come those people are floating?" What do you say? Hopefully it won't be "Because there is no gravity to hold them down." If you tell your child "The astronauts are floating because they are in a state of free fall around the Earth," you have provided the right answer but any child worth his or her salt is going to counter with "What's free fall?" This one might prove to be tougher to explain. Why not show your child exactly what free fall is.

The only materials that you will need to perform this experiment are a Styrofoam or plastic cup, something to poke a small hole in the cup, and a source of water. Some food coloring to add to the water would help make the experiment easier to observe. You will also need a location where you can conduct your experiment because it involves spilling water. If the weather is nice, you may want to consider going outside.

To explain free fall to your child, here is what you do:

Make a small hole in the side of the cup close to the bottom. Tell your child to pretend that the cup is a spaceship traveling around the Earth.
Have your child hold his or her finger over the hole and then fill the cup with water. The water can represent something or anything inside the spaceship cup. Ask your child to predict what will happen when he/she uncovers the hole in the cup and why.
Have your child take his/her finger away from the hole so that he/she can see the water running out of the cup. Make sure your child understands that it is gravity pulling the water out of the cup.
Fill the cup up again with water. This time ask your child what will happen when you uncover the hole and drop the cup at the same time. Will the water squirt out of the hole?
Drop the cup and note that the water does not squirt out of the hole.

What caused the water to stay in the cup? Well, because you dropped both the cup and the water, gravity caused both to fall together. You could say that the water was floating in the cup. Astronauts/water float inside the space shuttle/cup because both are falling together toward the Earth. With respect to the shuttle, a state of continuous free fall is achieved because of the shuttle's orbital motion around the Earth. For more information on this topic, refer to the article "The Missing Zero in Zero Gravity" in the October-December 1993 issue of PSF News.

This article originally appeared in the July-September 1994 issue of PSF News.

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