The force that holds the universe together
Drop a ball and it falls. Jump and you come back down. The Moon circles Earth and Earth circles the Sun. All of these happen because of gravity, a force of attraction between any two objects with mass.
Newton’s great idea
In 1687, Isaac Newton described gravity in his law of universal gravitation. The story says he was inspired by watching an apple fall. Newton realized that the same force that pulls an apple down also keeps the Moon in its orbit.
His law says gravity depends on two things:
| Factor | Effect on gravity |
|---|---|
| Mass | More mass → stronger pull |
| Distance | Farther apart → weaker pull (much weaker) |
If you double the distance between two objects, the force of gravity becomes four times weaker. This is called an inverse-square law.
Every object with mass pulls on every other object, even you and your pencil! But the pull is only noticeable when at least one object is enormous, like a planet.
Weight
Your weight is the force of Earth’s gravity pulling on your mass. On Earth, gravity pulls each kilogram with about 9.8 newtons of force.
weight = mass × 9.8 N/kg (on Earth)
| Place | Gravity compared with Earth | A 50 kg person’s weight |
|---|---|---|
| Earth | 1 | 490 N |
| Moon | About 1/6 | About 80 N |
| Mars | About 3/8 | About 185 N |
| Jupiter | About 2.5 times | About 1240 N |
Your mass stays the same everywhere. Only your weight changes.
Falling objects
In a vacuum (no air), all objects fall at the same rate: they speed up by 9.8 metres per second every second. This is called free fall.
- After 1 second: falling at 9.8 m/s
- After 2 seconds: 19.6 m/s
- After 3 seconds: 29.4 m/s
Galileo studied falling objects in the late 1500s and early 1600s and realized that heavy and light objects fall together when air doesn’t get in the way.
Air resistance
On Earth, air resistance pushes up against falling objects. A feather has a large surface for its tiny mass, so air slows it a lot. A rock is barely affected.
As a falling object speeds up, air resistance grows. Eventually it equals the pull of gravity, and the object stops speeding up. This top speed is its terminal velocity.
- A skydiver’s terminal velocity is about 200 km/h belly-down.
- Opening a parachute creates huge air resistance, slowing the skydiver to a safe landing speed of about 20 km/h.
Gravity in space
Orbits
Imagine throwing a ball very fast from a tall mountain. The faster you throw it, the farther it lands. Throw it fast enough (about 28 000 km/h near Earth) and the ground curves away beneath it as fast as it falls. It keeps falling around Earth forever. That’s an orbit!
The Moon, satellites and the International Space Station all orbit Earth. Earth and the other planets orbit the Sun.
Astronauts float
Astronauts on the space station float, but gravity there is still about 90% as strong as on the ground. They float because they and the station are falling around Earth together.
Tides
The Moon’s gravity pulls on Earth’s oceans, creating bulges of water. As Earth spins, coasts pass through these bulges, causing high and low tides. The Bay of Fundy, between Nova Scotia and New Brunswick, has the highest tides in the world, up to about 16 metres.
Black holes
A black hole has so much mass squeezed into so small a space that its gravity stops even light from escaping.
Einstein’s view
In 1915, Albert Einstein described gravity in a new way. In his theory of general relativity, mass bends space and time, like a heavy ball on a trampoline, and objects follow the curves. Einstein’s theory explains things Newton’s can’t, like light bending around stars. GPS satellites must correct for these effects to give accurate locations.
Common mistakes
- Thinking heavier objects fall faster. Without air, they fall together.
- Thinking there’s no gravity in space.
- Mixing up mass and weight.
Try it yourself
- Drop a coin and a flat piece of paper together. Then crumple the paper into a tight ball and try again. Explain.
- Make a parachute from a plastic bag and string. Test different sizes. Which falls slowest?
- Calculate your weight on the Moon and on Jupiter.
Quick recap
- Gravity attracts all objects with mass.
- More mass means more gravity; more distance means much less.
- Weight is the pull of gravity; mass doesn’t change.
- Without air, all objects fall at the same rate.
- Gravity keeps moons and planets in orbit and causes tides.