Gravity and orbits — Physics, 11–13 years
Gravity is the attraction between masses. Near Earth it pulls objects down, while in space it can bend an object’s path into an orbit instead of making it travel in a straight line.
A fall that keeps missing
Gravity pulls an orbiting object towards a planet, but the object is also moving sideways. It keeps falling towards the planet while the planet’s surface curves away beneath it. An orbit is therefore a continuous fall around a world, not a lack of gravity.
Why do satellites not fall straight down?
A thrown ball curves because gravity changes its direction. A satellite is moving so fast sideways that, as it falls, Earth curves away at nearly the same rate. This solved the puzzle of how the Moon can be pulled by Earth without crashing into it.
The Moon’s curved path
Imagine firing a ball horizontally from a high mountain. At 10 m/s it lands nearby because gravity pulls it down quickly. At 1000 m/s it travels much farther before falling. If its sideways speed is large enough, Earth curves away as fast as it falls, producing an orbit like the Moon’s.
There is gravity in orbit
People often say astronauts float because there is no gravity in space. This is understandable: they look weightless and are far above the ground. In low Earth orbit, gravity is still strong; astronauts float because they and their spacecraft are falling together around Earth.
Satellites and space missions
Navigation, weather forecasts and many communications use satellites in carefully chosen orbits. Engineers balance a satellite’s speed with the pull of gravity. The same idea helps plan journeys to the Moon and other planets, where a small change can alter the route greatly.
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