Key Concepts in Rotational Motion and Forces

Tangential speed

The linear speed tangent to a curved path, such as in circular motion.

Rotational speed

The number of rotations or revolutions per unit of time; often measured in rotations or revolutions per second or per minute. (Scientists usually measure it in radians per second.)

Centripetal force

A force directed toward a fixed point, usually the cause of circular motion: F = r

Centrifugal force

An outward force apparent in a rotating frame of reference. It is apparent (fictitious) in the sense that it is not part of an interaction but is a result of rotation with no reaction-force counterpart.

Rotational inertia

The property of an object to resist any change in its state of rotation: If at rest, the body tends to remain at rest; if rotating, it tends to remain rotating and will continue to do so unless acted upon by an external net torque.

Torque

The product of force and lever-arm distance, which tends to produce or change rotation: Torque = lever arm X force

Center of mass (CM)

The average position of the mass of an object. The CM moves as if all the external forces acted at this point.

Center of gravity (CG)

The average position of weight or the single point associated with an object where the force of gravity can be considered to act.

Equilibrium

The state of an object in which it is not acted upon by a net force or a net torque.

Linear momentum

The product of the mass of an object and its linear velocity.

Angular momentum

The product of a body's rotational inertia and rotational velocity about a particular axis.

Conservation of angular momentum

When no external torque acts on an object or a system of objects, no change of angular momentum can occur. Hence, the angular momentum before an event involving only internal torques or no torques is equal to the angular momentum after the event.