When an object moves in a circle at a constant speed, its direction constantly changes. A change in direction causes a change in velocity. This is because velocity is a vector quantity – it has an associated direction as well as a magnitude. A change in velocity results in acceleration, so an object moving in a circle is accelerating even though its speed may be constant.
An object will only accelerate if a resultant force acts on it. For an object moving in a circle, this resultant force is the centripetal force that acts towards the middle of the circle. Gravitational attraction provides the centripetal force needed to keep planets and all types of satellite in orbit.
|Example||Force provided by|
|Conker on a string||Tension in the string on the conker|
|Car on a roundabout||Friction from the road on the tyres|
|Satellite||Gravitational pull of the Earth on the satellite|
Artificial satellites travel in one of two different orbits:
Polar orbits take the satellites over the Earth's poles. The satellites travel very close to the Earth (as low as 200 km above sea level), so they must travel at very high speeds (nearly 8,000 m/s).
Geostationary satellites take 24 hours to orbit the Earth, so the satellite appears to remain in the same part of the sky when viewed from the ground. These orbits are much higher than polar orbits (typically 36,000 km) so the satellites travel more slowly (around 2,000 km/s).