Gravity, mass and weight

Gravity

The story goes that in 1665 or 1666, a young Isaac Newton watched an apple fall from a tree in his family's garden and wondered why it fell straight down. Nothing was pushing it. He realised that the Earth must be pulling it, and that the same pull might reach all the way to the Moon and hold it in orbit.

Gravity is a force of attraction between any two objects that have mass. It is a non-contact force that acts through a gravitational field, the region around an object where its gravity can be felt. Gravity:

• only ever attracts; it never pushes away
• is stronger for objects with more mass. Your pull on the Earth is real, but tiny; the Earth's pull on you is big because the Earth's mass is enormous.
• gets weaker with distance
• pulls everything towards the centre of the Earth. That's why "down" in Australia and "down" in England point in nearly opposite directions, and nobody falls off the bottom of the planet.

Gravity holds the atmosphere around the Earth, keeps the Moon orbiting us and the planets orbiting the Sun, and pulls rain down and rivers downhill. The Sun has so much mass that its gravity holds all eight planets, even Neptune, 4.5 billion kilometres away.

Mass and weight

These two words get mixed up all the time, but in science they mean different things:

To calculate weight, multiply the mass by the strength of gravity:

weight (N) = mass (kg) × gravity (N/kg)

On Earth, gravity pulls with 9.8 N on every kilogram (often rounded to 10). So a 50 kg student weighs 50 × 9.8 = 490 N. On the Moon, where gravity is 1.6 N/kg, the same student still has a mass of 50 kg but weighs only 80 N.

When you stand on bathroom scales, they actually measure your weight force pushing down, then show it as kilograms because they assume you're on Earth. Take them to Mars and they'd give the wrong answer.

Falling

When you drop something, gravity is an unbalanced force, so the object speeds up as it falls. If you throw a ball upwards, gravity is still pulling down the whole time: it slows the ball on the way up, stops it for an instant at the top, then speeds it up on the way down.

Without air, everything falls at the same rate. In 1971, astronaut David Scott dropped a hammer and a feather together on the Moon, which has no air, and they hit the ground at the same moment. On Earth, the feather floats down slowly because air resistance is large compared with its weight.

A skydiver shows how gravity and air resistance work together. Just after jumping, weight is much bigger than air resistance, so they speed up. As they go faster, air resistance grows, until it equals their weight. Now the forces are balanced and they fall at a steady top speed called terminal velocity, about 200 km/h. When the parachute opens, air resistance suddenly becomes much bigger than weight, so they slow down until the forces balance again at a gentle 15–20 km/h.

Buoyancy: why some things float

Drop a rock and a ping pong ball into a bucket of water. Gravity pulls on both, but the ping pong ball pops up. That's because water pushes up on anything in it with a force called buoyancy (or upthrust). If buoyancy is bigger than the object's weight, the object floats up; if weight is bigger, it sinks. Objects that are less dense than water (like wood, ice and ping pong balls) float. A friend in a swimming pool feels light because buoyancy is holding up most of their weight, even though their mass hasn't changed. Air has buoyancy too: helium balloons and hot-air balloons rise because they are less dense than the air around them.