Falling Around the Earth
- Physics
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- An orbit is a fall in which the ground curves away as fast as the object drops.
- Gravity weakens with the square of distance, and the same force holds the Moon in orbit.
- Astronauts are weightless because they are falling, not because gravity has stopped.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Newton’s cannon
Isaac Newton imagined a cannon on top of a very high mountain, firing a ball sideways. Gravity pulls the ball down, so it curves and lands some distance away. Fire it faster and it lands farther off. But the Earth is round, so the ground curves away beneath the moving ball. Fire it fast enough, about 7.9 kilometres per second near the surface, and the ground curves away just as fast as the ball falls. The ball keeps falling but never reaches the ground. It falls all the way round the Earth: an orbit. The International Space Station, about 400 km up, moves at about 7.7 km/s and goes round every 90 minutes or so. Faster than about 11.2 km/s, an object escapes altogether.
Weightless is not gravity-free
Astronauts float inside the space station, and many people think that means there is no gravity up there. It is not true. About 400 km up, gravity is still about 90% as strong as at the ground. The astronauts float because they and the station are falling together, both in the same orbit at the same speed, so nothing presses up on their feet. That is what weightless means. The Moon is falling around the Earth in the same way. It is about 60 times farther from the Earth’s centre than we are, and gravity there is about 3,600 times weaker, because gravity gets weaker with the square of the distance. Newton showed that this weaker pull is exactly what the Moon’s orbit needs.
Curriculum links: NGSS HS-PS2-4