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The straight line around which rotation takes place


When an object turns about an internal axis, or spin


When an object turns about an external axis

Linear Speed

The distance moved per unit of time

Tangential Speed

The speed of something moving along a circular path. The motion is always tangent to the circle

Rotational Speed

(Sometimes called angular speed) the number of rotations per unit of time

Centripetal Force

Any force that causes an object to follow a circular path
"Center seeking" or "towards the center"

Centrifugal Force

The outward force
"Center fleeing" or "away from the center"

Center of Gravity

Point at the center of an object's weight distribution, where the force of gravity can be considered to act.

Center of Mass

Often called center of gravity. The average position of all the particles of mass that make up an object

Unstable Equilibrium

The state of an object balanced so that any small displacement or rotation lowers its center of gravity

Stable Equilibrium

The state of an object balanced so that any small displacement or rotation raises its center of gravity

Neutral Equilibrium

The state of an object balanced so that nay small movement neither raises nor lowers its center of gravity

Lever Arm

The distance from the turning axis to the point of contact


The rotational analog of force; the product of a force and the lever arm (measured in newton meters). Tends to produce rotational acceleration.

Linear Momentum

Product of the mass and the velocity of an object. Also called momentum. (This definition applies at seeds much less than the speed of light.)

Angular Momentum

Product of rotational inertia and rotational velocity

Rotational Velocity

Rotational speed together with a direction for the axis of rotation or revolution

Law of Conservation of Angular Momentum

If no unbalanced external torques act on a rotation system, then the angular momentum of that system is constant.

Longitudinal Axis

The vertical axis that passes from head to toe. Rotational inertia about this axis is increased by extending a leg or arms.

Transverse Axis

When you perform a somersault or a flip. Tucking in your arms and legs reduces your rotational inertia about this axis. Straightening your arms and legs increase your rotational inertia about this axis.

Medial Axis

The front-to-back axis. You rate about this axis when doing a cartwheel.

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