Thomas J. Wdowiak is an Athena scientist who takes time out of his busy schedule to show kids how to do fun
science experiments at home. When he isn't teaching at the University of Alabama at Birmingham or training
at the Jet Propulsion Lab in California, he writes for the local newspaper in Birmingham in the "Just for
Kids" section. He goes by "Tommy Test Tubes," a nickname given to him when he was a kid. Now "Tommy Test
Tubes" contributes his experience and knowledge to the Athena web page with a new column. If you have a
question for Tom, click here.
Build Your Own Solar System!
Orbits! Planets, comets, and asteroids orbiting the Sun. The Moon orbiting the Earth. Spacecraft orbiting between planets. How does a
person come to understand all this? After all, didn't Isaac Newton have to invent Calculus, that mind-boggling branch of mathematics,
just to be able to explain it to himself? And of course he was a genius!
Well, there is a simple way to get the hang of what orbiting is all about and all you really need is a plastic paint bucket, a sheet of
rubber, a round stick, and some marbles. Doing it is no different than doing what ancient people did thousands of years ago - make a
"tom-tom" drum.
For gravity to be present there must be something scientists call mass, and as Albert Einstein showed, also energy. Matter, as exists
in stars (like the Sun), planets, and even you has both. Only thing is there has to be an awful lot of it! Well, there is a way to use
the Earth's gravity to simulate what happens in the solar system. First, gravity causes things to move downward (if things are free to
move). A ball (or in this experiment a marble) just sits still on a "flat" surface. If the surface is tilted it rolls because it is now
free to move downward. If you look from above the ball and surface, the view you have is of the ball moving sidewise. The only way you
can tell the ball is also going downward is by seeing it getting smaller in appearance.
Gravity makes things move toward the source of the gravity - the mass and energy of the star, planet, etc. Things fall inward!
Something (like a comet) falling toward the Sun can be "simulated" using the Earth's gravity, a piece of sheet rubber, and a round
stick (called a dowel). You also need the thing that falls - a marble. Stretching the sheet of rubber across the top of a paint bucket
gives you a flat surface. Put a marble on it and the marble just sits there. Push the rubber down in the center with a round stick of
wood or your finger, and the marble rolls. From above you see it move straight towards where the rubber has been pushed down. It looks
like it's being attracted to that point. Of course you know that because the rubber slopes inward from all directions (360 degrees), the
marble is moving downward because of Earth's gravity. But from above it looks like the stick (or your finger) is the cause.
In this experiment the sheet of rubber simulates space, a wooden stick represents the Sun, and a glass marble is a planet or comet. What
makes the experiment very interesting is that the shape of the rubber sheet when pressed down, the way it curves, is very similar to the
mathematical way gravity behaves. There is an important difference however between what you can do in this experiment and what is real for
space. There is friction between the rolling marble and the rubber. In empty space there is no friction to slow down a planet or comet.
After you stretch the rubber over the paint bucket according to the instructions I've written, you want to build a wooden frame to hold
the round wooden stick against the rubber. This lets you be free to concentrate on rolling the marble across the sheet rubber that simulates
"space". If your family or school has a video camcorder, it can be interesting to record your experimenting so you can review the results.
Tips on orbiting
First make sure you experiment is level! If not, adjust it. Press the rubber sheet down a little with the dowel (I painted mine at the end
with yellow marker to remind me it's the "Sun"), to press the rubber inward. If you push too far the rubber will tear and you'll have to go
through the work of putting on a new sheet. Be careful. About 1 or 2 centimeters is all you need to do. You can measure this on the stick
using the frame as a starting point when the stick just touches the rubber.
Try rolling the marble gently. If you don't, you'll end up looking for it somewhere on the floor! You want the roll to miss hitting the stick.
Probably the first time the marble will roll toward the stick, curving around, and roll away toward the rim (and fall off if you don't catch it).
That kind of orbital path has a special name - "hyperbolic", from the geometric curve hyperbola. The closest distance to your "sun" is called
the perihelion distance.
With a little practice you will be able to get the marble to roll toward the "Sun", pass around it (perihelion), go out to a point (called
aphelion), and roll back for another trip around. Congratulations, you have made a closed orbit, like the Earth's. The shape of this kind of
orbit also has a specific name - "elliptical", after the geometric curve called an ellipse. It is not a circle! You can make a circular orbit,
it's not easy to do, if the marble is rolled just right. While rocket scientists are able to put satellites orbiting the Earth in circular
paths; Nature almost never does do this! Even the Moon's orbit isn't circular; it's an ellipse.
One last thought, not only did Einstein realize both mass and energy caused gravity, he came to understand that gravity is really the shape of
space. The presence of the Sun changes the shape of space just like pressing the stick (or you finger) changes the shape of the rubber from
flat to non-flat or curved. During June 2004, my scientist friends and I are going to use the shape of the space of the Solar System to send
two robot explorers toward the planet Mars. They will have their own orbits, and when they reach the orbit of Mars, the planet will also be
there.
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