Orbits and speed - Higher

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

A circle of dotted lines orbiting a point counterclockwise. This shows the velocity and effects of centripetal force.

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


The moon orbits the Earth and shows the force of gravity on the moon, the direction the moon would travel without Earth's gravity, and the moon's actual orbit.

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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