# Orbits and speed - Higher

When an object moves in a circle at a constant , its direction constantly changes. A change in direction causes a change in . This is because velocity is a quantity ā it has an associated direction as well as a magnitude. A change in velocity results in , so an object moving in a circle is accelerating even though its speed may be constant.

An object will only accelerate if a acts on it. For an object moving in a circle, this resultant force is the that acts towards the middle of the circle.

### Example

Gravitational attraction provides the centripetal force needed to keep a planet in orbit around the Sun, and a satellite in orbit around a planet. For example, gravitational attraction between the Earth and the Moon keeps the Moon in orbit around the Earth. An object moving in a circular orbit at a constant speed has a changing velocity. This is because velocity is a vector quantity that depends on speed and direction. The object in orbit is accelerating, even though its speed remains constant, because its velocity is changing.

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