Orbits and gravity
Fire Newton’s cannon faster and faster until the ball falls all the way around the Earth, and see why gravity weakens with distance.
Standards: HS-PS2-4
1Wonder
Astronauts on the space station float around as if there were no gravity. But the station is only about 400 km up, where gravity is still almost as strong as on the ground. So why doesn’t it fall?
2Explore
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Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.
- Fire at 5 km/s, then faster and faster. What happens to where the ball lands?
- Find a speed where the ball never lands. Is its path a circle?
- Try 9.5 km/s. What shape is the path now? Where is the ball slowest?
- Watch the red arrow and the readout. How does the pull change when the ball is twice as far from the center?
- What is the slowest speed that makes the ball escape?
3Explain
Throw harder, land farther
Newton imagined a cannon on a very tall mountain. The faster the ball is fired sideways, the farther around the Earth it lands, because the ground curves away beneath it.
Every two masses attract. An orbit is a fall that keeps missing the ground: the object moves sideways fast enough that the Earth curves away beneath it.
4Practice
A satellite moves to twice its distance from the Earth’s center. How strong is the pull now?
5Recap
- Gravity pulls every two masses together: F = G m₁ m₂ / r².
- Twice as far → ¼ the pull; three times as far → 1/9.
- An orbit is falling around a planet while moving sideways fast enough to keep missing it.
- Near the Earth: about 7.9 km/s for a circular orbit, 11.2 km/s to escape.
- Faster orbits stretch into ellipses.