Physics A2 · Motion in a circle. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A body moving in a circle at constant speed is still accelerating, because the direction of its velocity changes all the time. This centripetal acceleration points towards the centre of the circle.
It is caused by a resultant force of constant magnitude that is always perpendicular to the velocity. This force has no component along the direction of motion, so it does no work. The speed and kinetic energy stay the same; only the direction changes.
Centripetal force is not an extra force. It is the name for the resultant of the real forces (tension, friction, weight, normal contact force) acting towards the centre. If that force disappears, the body moves off along the tangent.
Students add an outward 'centrifugal force' to balance the inward force. Do not: the forces are not balanced. The resultant force towards the centre is what equals mv2/r.
Answer: About 95 N towards the centre of the roundabout.
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