Orbits and speed - Higher
Orbits and constant speed
When an object moves in a circle at a constant The distance travelled in a fixed time period, usually one second., its direction constantly changes. A change in direction causes a change in velocity. This is because The speed of an object in a particular direction. is a A vector describes a movement from one point to another. A vector quantity has magnitude (size) and direction. 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 梅 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.
Orbits and changing speed
The gravitational attraction between two objects decreases with distance. This means that the closer the two objects are to each other, the stronger the force of gravity between them. If the force between them is greater, a greater acceleration will occur.
The greater the acceleration, the greater the change in velocity - this causes the object to move faster. This means that objects in small orbits travel faster than objects in large orbits.
The graph shows how the orbital speed of a planet changes with its distance from the Sun.
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鈥檚 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 3 km/s).