Question? What keeps a satellite up in orbit? What prevents it from falling out of the sky?

Answer. Nothing! It is falling! It just keeps missing the earth. (?)

Your text book (p. 164) describes a famous "thought experiment", known as Newton's Cannon, that was put forward by Isaac Newton to help explain the orbit of a satellite. The following applet lets you experiment with Newton's Cannon.

Imagine firing a cannon from the top of a high mountain. The cannon ball falls to the earth under the influence of gravity. When the initial speed of the cannon ball is low, the ball falls to the earth as expected. However, there is a certain speed at which the cannon ball falls toward the earth at the same rate as the earth's surface curves away. The cannon ball then "misses" the earth and keeps "falling around it", i.e. orbiting the earth.

In this applet you can adjust the initial speed of the cannon ball using the slider. Experiment with different initial velocities and observe their effect. As you do so, try to answer the following questions. The answers can be found further down this page.

1. What initial velocity is needed to make the cannon ball follow a circular path (the inner black circle) around the earth?
2. What happens to the cannon ball if its initial velocity is faster than needed to make it follow a circle?
3. What initial velocity is needed to make the cannon ball just reach the outermost circle?
4. When the ball does reach the outermost circle, it doesn't stay there. It immediately begins to fall back closer to the earth. What would have to happen if we were to keep the ball moving along this higher circular orbit?

Here are the answers to the earlier questions. If you didn't get them right, experiment with the applet again, paying attention to the things described here.

1. At about 16,200 mph, give or take a little.
2. Basically it goes into an elliptical orbit (rather than a circular one) with the launch point (the mountain top) as the closest point to the earth in the orbit. The faster the velocity, the more eccentric the orbit. If the velocity is very high, it will escape earth's gravity altogether and travel off into space. (You will hear a sound effect to indicate when this happens. If you don't hear it, just wait and the ball will return into the field of view - remember, it travels more slowly when it is further from the earth.)
3. At about 17,000 mph it just reaches the higher altitude circle at its furthest point from the earth.
4. It falls back to earth if it is going too slow. Therefore, if we want to put the ball into the higher circular orbit we would have to give it a greater speed just as it reaches that altitude. It's path would then be less curved, i.e. it is in a higher altitude orbit. (We can't do this in this simulation.)