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By the end of these notes, you should be able to:
Think about what Newton's First Law tells us: an object will keep moving in a straight line at constant speed unless a force acts on it.
So if an object is moving in a circle — constantly changing direction — there must be a force acting on it. Without that force, the object would fly off in a straight line (called a tangent to the circle — a straight line that just touches the circle at one point).
Example: Imagine spinning a ball on a string above your head. The string pulls the ball towards your hand. The moment you let go, the ball flies off in a straight line. The tension in the string was the force keeping it moving in a circle.
Example: The Earth orbits the Sun because gravity pulls the Earth towards the Sun. Without gravity, the Earth would shoot off into space in a straight line.
The force that keeps an object moving in a circle is called the centripetal force.
Centripetal force = a resultant (net) force that always points towards the centre of the circle.
The word "centripetal" just means "centre-seeking." It describes the direction of the force, not what type of force it is. The centripetal force can be:
| Situation | What Provides the Centripetal Force |
|---|---|
| Ball on a string | Tension in the string |
| Earth orbiting the Sun | Gravitational pull of the Sun |
| Car going round a bend | Friction between tyres and road |
| Electron orbiting a nucleus | Electrostatic (electric) force |
Important: Centripetal force is not a separate, special type of force. It is simply the name we give to whichever force (or combination of forces) is pointing towards the centre of the circle.
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