Concept preview

What you will be able to do

1. 01Derive orbital speed by setting gravity equal to the centripetal force
2. 02Apply Kepler's third law relating period and radius
3. 03Explain geostationary orbits and apparent weightlessness

A satellite in orbit is doing something remarkable — it is continuously falling toward Earth, but moving sideways so fast that the ground curves away beneath it just as quickly. Newton imagined firing a cannonball ever faster from a mountaintop: fire it hard enough and it falls all the way around the planet. That is an orbit — perpetual free fall. Gravity is not balanced by anything; it is the unbalanced centripetal force that bends the straight-line path into a closed loop.