Circular motion – Higher
When an object moves in a circle at a constant The distance travelled in a known time period, eg miles per hour or kilometres per hour. , its direction constantly changes. A change in direction causes a change in The speed of an object in a particular direction.. This is because velocity is a A physical quantity that has both magnitude (size) and direction. Eg force, velocity, displacement, acceleration. quantity – it has an associated direction as well as a magnitude. A change in velocity results in The rate of change in speed (or velocity) is measured in metres per second squared. Acceleration = change of velocity ÷ time taken., so an object moving in a circle is accelerating even though its speed may be constant.
An object will only accelerate if a The single force that could replace all the forces acting on an object, found by adding these together. If all the forces are balanced, the resultant force is zero. acts on it. For an object moving in a circle, this resultant force is the Force, needed for circular motion, which acts towards the centre of a circle. 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 |
| Example | Conker on a string |
|---|---|
| Force provided by | Tension in the string on the conker |
| Example | Car on a roundabout |
|---|---|
| Force provided by | Friction from the road on the tyres |
| Example | Satellite |
|---|---|
| Force provided by | Gravitational pull of the Earth on the satellite |
Artificial satellites travel in one of two different orbits:
- polar orbits
- A satellite orbiting a planet at the same rate as the planet. A geostationary satellite orbiting Earth has a period of 24 hours. 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).
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