Every object with mass pulls on every other object with mass. This pull is gravity. It is stronger when the masses are bigger and weaker when they are farther apart. Earth's huge mass is why things fall; the Sun's far larger mass holds all the planets.
Key idea
Gravity pulls masses toward each other. An orbit is falling around a body while moving sideways so fast that you keep missing it.
Why it works
Imagine a cannon on a very tall mountain. Fire a ball slowly and it curves down and lands. Fire it faster and it lands farther away. Earth is round, so the ground curves away under the ball too. Fire it fast enough (near Earth, about 8 km every second) and the ground curves away exactly as fast as the ball falls. The ball is still falling, but it never lands: it is in orbit. The Moon does the same around Earth, and Earth around the Sun.
Worked example: Weight on Mars
Gravity at the surface of Mars is about 0.38 of Earth's. A rover weighs 9,000 N on Earth. What does it weigh on Mars, and what happens to its mass?
- Weight is the pull of gravity, so it shrinks with the weaker gravity: $9{,}000 \times 0.38 = 3{,}420$ N.
- Mass is the amount of matter. The rover is made of the same stuff, so its mass does not change.
Answer: About 3,420 N; the same mass
Worked example: Farther away
A satellite moves from 1 Earth radius from Earth's center (on the ground) to 2 Earth radii. How does Earth's pull on it change?
- Gravity weakens with the square of the distance.
- Twice as far: $2^2 = 4$, so the pull is $\tfrac{1}{4}$ as strong.
Answer: One quarter as strong
Common mistake
Believing there is no gravity in space, so astronauts float. The space station orbits only about 400 km up, where gravity is still about 90% as strong as on the ground. Astronauts float because they and their station are falling around Earth together, like riders in a falling elevator. Check: without gravity, the station would fly off in a straight line instead of circling Earth.
Launch your own satellites in the Orbit Lab.