Orbits and speed/velocity - 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.A circle of dotted lines orbiting a point counter-clockwise. 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.

Example

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.The Moon orbits the Earth - the diagram 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 planets in orbit around the Sun and all types of satellite in orbit around the Earth. The Earth's gravity keeps the Moon orbiting us. It keeps changing the direction of the Moon's velocity. This means gravity makes the Moon accelerate all the time, even though its speed remains constant.

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