Centripetal Force and Circular Motion

An object moving in a circle at constant speed is continuously changing direction. Because velocity is a vector (speed and direction), a change in direction means a change in velocity, which means the object is accelerating.

Centripetal force

The force that causes this acceleration is called the centripetal force. It always acts toward the centre of the circle. Without this force, the object would travel in a straight line (Newton’s first law).

Centripetal force is not a new type of force. It is the name given to whatever force provides the inward pull in each situation.

Situation	Source of centripetal force
Planet orbiting the Sun	Gravitational force
Car turning a corner	Friction between tyres and road
Object on a string whirled in a circle	Tension in the string
Electron orbiting a nucleus	Electrostatic force

What changes the centripetal force needed?

The centripetal force required increases when:

• the mass of the object increases
• the speed of the object increases
• the radius of the circle decreases

What happens if the force disappears?

If the centripetal force suddenly stops (for example, a string breaks), the object continues in a straight line tangent to the circle at the point where the force was removed. It does not fly outward.

Common errors and how to correct them

Saying the object “flies outward” when released. The object moves in a straight line tangent to the circle, not radially outward. There is no outward force on the object.

Describing centripetal force as a separate force alongside tension, gravity, or friction. Centripetal force is not an additional force. It is the resultant force directed toward the centre, provided by one of the real forces (gravity, tension, friction).

How to apply this in an exam

When asked to identify the centripetal force, name the actual force (gravity, friction, tension) rather than just writing “centripetal force.” State its direction as “toward the centre of the circle.”