Revision notes · Physics
Our solar system has the Sun (a star) at its centre, orbited by 8 planets, dwarf planets, moons, asteroids and comets.
The planets
Inner rocky planets: Mercury, Venus, Earth, Mars.
Outer gas giants: Jupiter, Saturn, Uranus, Neptune.
Orbits
Gravity provides the force that keeps planets orbiting the Sun and moons orbiting planets.
The further from the Sun, the longer a planet takes to orbit and the slower it moves.
Circular motion and acceleration (Higher)
An object moving in a circle at a constant speed is still accelerating.
Velocity is a vector — it has a size (speed) and a direction.
In an orbit the direction keeps changing, so the velocity keeps changing — and a changing velocity is an acceleration.
The acceleration is towards the centre of the orbit, caused by the force of gravity between the orbiting body and the object it orbits.
For a stable orbit, if the radius gets smaller the speed must get bigger (and the other way round).