Gravity & Orbits

Drop a ball and it falls. Jump, and you come back down. That pull is gravity — the same force that keeps the Moon circling Earth and Earth circling the Sun. It’s one of the quietest forces in the universe, and one of the most powerful.

What gravity is

Gravity is a pull between anything that has mass — that is, anything made of stuff. You pull on the Earth and the Earth pulls on you. Two simple rules decide how strong that pull is:

• More mass means a stronger pull. A planet pulls harder than a pebble.
• Closer means a stronger pull. Move twice as far away and the pull gets much weaker.

You don’t notice yourself pulling on the Earth because you have so little mass compared to a whole planet. But the Earth’s huge mass pulls on you all the time — that’s your weight pressing you into your chair right now.

Why things fall — but the Moon doesn’t

Here’s the puzzle: gravity pulls the Moon toward Earth just like it pulls an apple toward the ground. So why does the apple hit the grass while the Moon keeps sailing around us, year after year?

The answer is one of the great “aha!” ideas in science: an orbit is just falling sideways, fast enough to keep missing the ground.

Imagine throwing a ball. Throw it gently and it curves down and lands nearby. Throw it harder and it lands further away, because the ground curves away beneath it as it falls. Now imagine throwing it so fast that the Earth curves away just as quickly as the ball falls. The ball keeps falling — but it never gets any closer to the ground. It’s in orbit.

The Moon is doing exactly that. It’s constantly falling toward Earth, but it’s also moving sideways so fast that it keeps missing. Falling forever, in a circle.

Play with gravity

Time to build some orbits yourself. There’s a heavy star in the middle.

• Click near the star: the body drops straight in — that’s just falling.
• Click out wide and drag sideways before you let go: the drag sets the launch speed and direction. Find a speed where the body keeps looping around without crashing or flying away — you’ve made an orbit.
• Slide the star mass up: watch the pull get stronger, orbits tighten, and slow bodies plunge inward.

Notice the balance: too slow and gravity wins, dragging the body into a spiral. Too fast and the body escapes and never returns. A stable orbit lives right in between.

Mass vs. weight (they’re not the same)

People use these words to mean the same thing, but to a physicist they’re different:

• Mass is how much stuff you’re made of. It never changes.
• Weight is how hard gravity is pulling on that mass right now. It changes with where you are.

On the Moon, whose gravity is about one-sixth of Earth’s, you’d have the exact same mass — but you’d weigh about six times less, which is why astronauts bounce around. Same you, weaker pull.

Escape velocity

If a slow body falls back and a fast one orbits, what about a really fast one?

Throw something fast enough and it can break away from a planet’s gravity completely and never come back. The speed you need to do that is called the escape velocity. For Earth it’s about 11 kilometres per second — roughly 40,000 km/h. That’s why rockets have to be so powerful: they’re racing to beat Earth’s grip.

Escape velocity depends on the body you’re escaping: more mass (or being smaller and denser, so you can get closer to the middle) means a higher escape velocity. The stronger the gravity, the faster you have to go to get away.

Check yourself

You now have the tool you need for the strangest object in the universe: if escape velocity can climb higher and higher as gravity gets stronger, eventually it passes the speed of light — and that’s where black holes begin.