5.6 - Circular Motion

Circular motion needs a centripetal force

• Velocity is both the speed and the direction of an object.
• An object moving around a circle at constant speed is constantly changing direction, so its velocity is constantly changing.
• A changing velocity means the object is accelerating.
• If it's accelerating, there must be a resultant force acting on it.
• This force acts towards the centre of the circle, perpendicular to the direction of the velocity.
• The force that keeps an object moving in a circle is called the centripetal force.

How centripetal force depends on speed, radius and mass

The centripetal force needed to keep an object moving in a circle increases if:

• the object's mass increases
• the object's speed increases
• the radius of the circle decreases (the circle gets smaller)