The Force Behind Circular Motion
Something has to pull inward. Centripetal force is a job, not a new kind of force.
Lesson at a glanceShow the key points
- In an inertial frame, a body in uniform circular motion needs a net force mv²/r = mω²r toward the centre.
- 'Centripetal force' names a direction (toward the centre), not a new kind of force.
- It must be supplied by tension, gravity, friction, a normal force, an electric force or a combination.
- Remove the inward force and the body moves off along the tangent, not outward along the radius.
Every circling object is accelerating toward the centre, so by Newton’s second law something must be pulling it there. The only question is: what?
The force circular motion requires
Seen from an inertial frame, a particle moving in a circle is accelerating, and acceleration needs a net force. For uniform circular motion that acceleration is toward the centre, so the net force must point toward the centre too, with
Because this net force points to the centre, it is called the centripetal force (“centre-seeking”).
A 200 g block slides at steady speed round a horizontal circular groove of radius 40 cm with vertical side walls. It takes 1.6 s per lap. What force does the outer wall exert on it?
- 1Uniform circular motion needs a net inward force .
- 2Centripetal force is a role played by real forces (tension, gravity, friction, normal force), not an extra force.
- 3Without it, the body leaves along the tangent.
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