Materials

• Sheet
• Garbage can
• About three metres of heavy string
• Baseball
• Golf balls
• Cardboard tube from a roll of paper towels

Instructions

1. Fit the sheet over the top of the garbage can. Place it like a drum, tying it down with the string.

2. Place the baseball in the centre. (The baseball should be forcing the sheet inwards a bit. Have an assistant push down on it slightly if needed.) Place a golf ball near the edge. This is a "satellite" in orbit about a planet. What happens is similar to what happens when a non-moving object is dropped high above the Earth.

3. Now, launch the satellite into "orbit". Raise the cardboard tube slightly, (if the golf ball doesn't fit, cut the tube down its length) and let the golf ball roll down the tube's inside to gain speed. Observe the path of the "satellite" closely.
   
   
4. By trial and error, see if you can determine what elevation is needed on the cardboard tube to have the marble establish a circular orbit. What happens if the velocity of the marble is too great? Too small? Try aiming the marble at various angles relative to the edge of the garbage can. What happens to the marble when it is launched toward the ball in the middle? Try to aim the marble so that it will stay in "orbit" for the longest time.

What's Happening?

According to a physical law first stated by the German scientist Johannes Kepler (1571-1639), the orbit of any planet around the Sun is elliptical. This law is called Kepler's first law. A circle is a special case of an ellipse. How did you produce a circular orbit in the above experiment? Using the apparatus, now determine what conditions will produce a non-circular orbit.

One problem faced by space scientists is adjusting orbits to higher or lower altitudes. Often one orbital vehicle needs to rendezvous with another object in another orbit. This can be done by changing the orbital velocity, but as you have seen, this also changes the shape of the orbit.