Orbits and speed - Higher
When an object moves in a circle at a constant The distance travelled in a known time period, eg miles per hour or kilometres per hour. , its direction constantly changes. A change in direction causes a change in The speed of an object in a particular direction.. This is because velocity is a A vector describes a movement from one point to another. A vector quantity has magnitude (size) and direction. quantity 鈥 it has an associated direction as well as a magnitude. A change in velocity results in The rate of change in speed (or velocity) is measured in metres per second squared. Acceleration = change of velocity 梅 time taken., so an object moving in a circle is accelerating even though its speed may be constant.
An object will only accelerate if a The single force that could replace all the forces acting on an object, found by adding these together. If all the forces are balanced, the resultant force is zero. acts on it. For an object moving in a circle, this resultant force is the Force, needed for circular motion, which acts towards the centre of a circle. 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.