In uniform circular motion the centripetal acceleration points towards the centre of the trajectory. By Newton’s second law, if there is acceleration there must be a force: there must therefore exist a net force directed towards the centre.
Principle — Centripetal force
The resultant of the forces on a body in circular motion has a component towards the centre (centripetal) equal to:
The most important point, and the most often misunderstood, is this: centripetal force is not a new force. It is the resultant of forces already present in the problem — the tension of a string, gravity, friction, the constraint reaction, the elastic force of a spring. “Centripetal” is a role, not a type of force: it describes the direction the resultant must point in, not a new physical interaction. In every circular-motion problem the right question is: which real force is providing the push towards the centre?
Centrifugal force does not exist (in inertial frames)
“Centrifugal force” does not exist in an inertial reference frame. The sensation of being “pushed outwards” on a bend is an effect of inertia: the body tends to carry on in a straight line (Newton’s first law), and it is us — or the car door — pushing it towards the centre to force it to turn. A direct illustration of this is the cut string experiment: the stone flies off tangentially, not radially.
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Topics: Dinamica Concepts: Forza centripeta · Accelerazione centripeta · Seconda legge di Newton · Moto circolare uniforme
Related exercises: Why circular motion is accelerated · True or false on circular motion and the inclined plane · The bucket of water in the loop-the-loop