Gravity: the pull that holds the world together
Mass and weight, why everything falls equally fast without air, parachutes, tides and floating astronauts. With a fair-test parachute experiment.
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Everything that has mass (everything made of stuff) pulls on everything else. You and your desk pull on each other too, but that pull is far too small to feel. The Earth has an enormous mass, so its pull on you is strong. The more mass, the stronger the pull; the further away, the weaker it gets.[1]
Mass is how much stuff you are made of, and it stays the same everywhere. Weight is how hard gravity pulls on you. On the Moon gravity is about 1.6 m/s², compared with 9.8 m/s² on Earth, so you would weigh about one sixth as much. On Mars (3.7 m/s²) you would weigh a bit more than a third, and on Jupiter (23.1 m/s²) almost two and a half times as much.[2][1]
Long ago people thought heavy things fall faster. In 1586 the Flemish engineer Simon Stevin and his friend Jan Cornets de Groot dropped two lead balls of different weights from a church tower in Delft. They landed at almost the same time. (A famous story says Galileo did this from the Leaning Tower of Pisa, but there is little proof that he did.)[10][11]
Without air, everything falls equally fast, whatever its mass. In 1971 the Apollo 15 astronaut David Scott tested this on the Moon, where there is no air: a hammer and a falcon feather hit the ground together.[6][3]
On Earth, air pushes back on falling things. This is called air resistance. Big, flat shapes catch more air, which is how a parachute works. Gravity pulls just as hard on the parachute; the air just pushes back harder.[6]
Gravity reaches very far. The Earth's gravity keeps the Moon in its orbit, and the Moon's gravity pulls on the oceans and causes the tides. The Sun's gravity keeps the Earth and the other planets going around it.[1][5]
Astronauts float in their space station, but not because there is no gravity up there. At about 400 km high, gravity is still almost 90% as strong as on the ground! The station and everyone in it are falling around the Earth together, so they float.[4]
Try it: The parachute test
You need
- A thin plastic bag or tissue paper
- Thread or thin string, tape and scissors
- The same small weight for each parachute (for example 3 paper clips or a lump of modelling clay)
- A stopwatch (or a phone)
- A tape measure
Safety
Drop from standing height with arms raised, or let the teacher drop from a safe, stable spot. Never climb on chairs or tables. Take care with scissors.
What to do
- Cut three squares from the plastic: 10 cm, 20 cm and 30 cm wide.
- Tape a 25 cm thread to each corner, and tie the four threads to the same weight.
- Choose one drop height (for example 2 metres, measured). Always drop from that height.
- Drop each parachute three times and time how long it takes to reach the floor.
- Work out the average time for each size. Which parachute falls slowest?
What happens, and why?
Gravity pulls on each parachute with the same force, because the weight is the same. The bigger canopy pushes against more air, so the air resistance is bigger and the parachute falls more slowly. This is a fair test: you changed only one thing (the size) and kept the weight and the height the same.[6]
Play: Mystery world
Something is dropped from 10 metres on a mystery world. Watch how fast it falls, then guess the world. Earth, Moon, Mars or Jupiter?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16