How gravity keeps us on Earth — Earth science, 7–10 years
Gravity is the pull between objects with mass, and Earth’s large mass pulls us toward its centre.
A pull you cannot see
Gravity is a pull between things that have mass. Earth is so massive that it pulls people, air, water and objects toward its centre. That is why a dropped ball falls down and why your feet stay on the ground instead of floating away.
The problem of falling things
People needed one idea that could explain falling apples, oceans staying near Earth and the Moon remaining in space. Gravity links these events: masses attract one another, and the stronger pull of Earth keeps nearby things close. Without this idea, each falling or orbiting object would seem like a separate mystery.
Dropping a ball
Hold a 200-gram ball one metre above the floor. At first, your hand pushes up and balances Earth’s downward pull. Let go: the upward push disappears, but gravity remains, so the ball accelerates downward. When it meets the floor, the floor pushes up and stops it.
Heavier does not always fall first
It seems sensible that a heavy stone should fall faster than a light feather because the stone is pulled more strongly. The missing part is air resistance: air holds back the feather much more. In a vacuum, where there is no air, a stone and a feather fall together.
Gravity in everyday life
Gravity lets rain fall, rivers flow downhill and buildings stand on foundations. Engineers include it when designing bridges, lifts and playground equipment. Astronauts still feel Earth’s gravity in orbit, but they are falling around Earth together with their spacecraft, so they appear weightless.
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