Orbital Motion (GCSE Physics)

Orbital Motion

Orbits

Circular Orbits

As we have mentioned before, velocity is a vector. It has both size and direction, meaning that the speed of movement and the direction of movement are important.

Since velocity is determined by two different factors, the velocity of objects in orbit can be changed in two ways:

• Direction of orbit – if the direction of an object changes, then the velocity of the object will change. The force of gravity keeps objects in their orbits. Gravity can change the direction but not the speed of a planet. The force of gravity is acts at right angles to the direction of travel.
• Speed of orbit – if the speed of the planet changes, then the velocity will change. However, the force of gravity is unable to change the speed of a planet, so planets have constant speed of orbit.

Stable Orbits

There is a centripetal force which keeps planets moving in a circle, since it acts towards the centre of the circle (perpendicular to the direction of motion).

The centripetal force can produce an acceleration towards the centre of the object. In a stable orbit, the speed of the object is affected by the radius of the orbit. When the speed of the orbit changes, the radius of the orbit must also change.