Centrifugal Force

Outward radial fictitious force experienced by objects in rotating frames of reference.

General idea of the topic

The centrifugal force is the inertial force that appears for an observer rotating with a frame, for example on a platform or inside a car on a bend. It points away from the rotation axis and equals mass times the square of the angular velocity times the distance to the axis.

It is the fictitious force most often confused with the centripetal one. From an inertial frame it does not exist: the body tends to continue in a straight line and the only thing acting is the real centripetal force. From the rotating frame the equilibrium of a body at rest relative to it is explained by balancing the real inward force with this outward force.

What the student is expected to understand

By the end, the student should be able to compute the centrifugal force in a frame rotating at constant angular velocity and use it to solve relative-equilibrium problems, such as the tilt of a conical pendulum or the force on a passenger in a bend.

The student should also know why centrifugal and centripetal forces must not be added in the same diagram.

Recommended learning route

1. Definition of Centrifugal Force: defines the centrifugal force from the rotating frame and fixes its direction, value and origin.
2. Examples of Centrifugal Force: solves examples: objects on a platform, centrifuges and vehicles on a bend, checked from the inertial frame.

Physical criterion to acquire

The essential criterion is to work in a single reference frame. In the inertial one only the real centripetal force exists; in the rotating one there is a centrifugal force that balances the real one. Mixing both descriptions, or drawing both forces at once, duplicates the effect and leads to incorrect equations.

Connection with real problems

• road bends: what the passenger feels.
• centrifuges and washing machines.
• rotating rides and space stations with artificial gravity.
• separation of mixtures by density.

Final goal of the topic

The final goal is to compute and interpret the centrifugal force in a rotating frame and contrast the result with the description from an inertial frame.