Orbital motion
Gravity provides the force needed to maintain stable orbit of planets around a star and also of moons and artificial satellites around a planet.
Explaining orbits
For an object to remain in a steady, circular orbit it must be travelling at the right speed. The diagram shows a satellite orbiting the Earth.
There are three possible outcomes:
- If the satellite is moving too quickly then the gravitational attraction between the Earth and the satellite is too weak to keep it in orbit. If this is the case, the satellite will move off into space. This occurs at speeds around or above 11,200 metres per second (m/s).
- If the satellite is moving too slowly then the gravitational attraction will be too strong, and the satellite will fall towards the Earth. This occurs at speeds around or below 7600 m/s.
- A stable orbit is one in which the satellite’s speed is just right - it will not move off into space or spiral into the Earth, but will travel around a fixed path.
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 velocity 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 ÷ 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. In order to change orbital speed, an object must change the radius of its orbit at the same time, to maintain a stable orbit.
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 3 km/s).