Newton's Law of Universal Gravitation, Explained

Here's how gravity actually works — from a falling apple to why the tides come in twice a day.

Newton's law of universal gravitation

The law of universal gravitation states that every object with mass exerts an attractive force on every other object with mass — a force proportional to the product of their masses and inversely proportional to the square of the distance between them. Newton's insight was that an apple falling to the ground and the moon orbiting the Earth are both explained by this same single force: gravity.

The difference between weight and mass

Mass is an object's inherent amount of matter, and it stays the same no matter where the object is. Weight, on the other hand, is the force of gravity acting on that object, so it changes wherever the local gravitational acceleration differs. That's why the same person weighs much less on the moon than on Earth, even though their mass hasn't changed at all.

Why tides happen

The moon's gravitational pull draws the ocean water on the side facing the moon toward it, creating high tide there, and because the Earth is rotating, a corresponding high tide also occurs on the opposite side — which is why there are two high tides and two low tides roughly every day. The sun's gravity affects tides too, and when the sun and moon line up, the difference between high and low tide is at its largest — a phenomenon called a spring tide.

How a satellite stays in orbit

A satellite is constantly falling toward Earth, but it's also moving sideways so fast that it keeps 'falling' along a curved path that matches the curvature of the Earth's surface, maintaining a roughly circular orbit. Earth's gravity acts as the centripetal force that holds the satellite on that circular path, which is why it keeps orbiting the Earth instead of crashing into the surface.

Why gravity weakens sharply with distance

Because gravitational attraction is inversely proportional to the square of the distance, if the distance between two objects doubles, gravity drops to a quarter of its original strength; if it triples, gravity drops to a ninth. Thanks to this inverse-square law, an astronaut on a space station far from Earth experiences a state that feels close to weightlessness.

What is escape velocity?

Escape velocity is the minimum speed needed to break completely free of a celestial body's gravity into space — it's larger the more massive the body is and the smaller its radius is. Earth's escape velocity is about 11.2 km per second, and a rocket has to reach roughly that speed to break free of Earth's gravitational pull.

Why the law of universal gravitation matters

The law of universal gravitation is a landmark theory in the history of physics — it revealed that an apple falling from a tree and the moon orbiting the Earth are, in fact, caused by the exact same force: gravity. Thanks to this law, we can predict everything from planetary motion to satellite orbits using a single unified principle.

Orbital motion and conservation of angular momentum, too

The reason a planet's or satellite's orbital speed changes depending on its position in orbit can also be explained by the law of conservation of angular momentum, since that principle governs how rotational motion is conserved.

Frequently Asked Questions

Does gravity act the same way everywhere in the universe?

No. The strength of gravity depends on the masses of the two objects involved and the distance between them, so gravity weakens sharply — inversely proportional to the square of the distance — the farther apart they are. That's why, far enough away from Earth in space, Earth's gravitational influence becomes so small it's barely felt.

Are astronauts on a space station weightless because there's no gravity there?

No. At the altitude of the International Space Station, Earth's gravity is still about 90% as strong as it is at the surface. Astronauts feel weightless because the station is continuously falling toward Earth while also moving sideways very fast, keeping it in a kind of perpetual free fall as it orbits.