B.2.1.1 Angular Motion and Momentum

Angular motion

Rotational movement around an axis

Production of angular motion

Occurs when a force is applied at a distance from the centre of mass

Eccentric force

A force whose line of action does not pass through the centre of mass

Effect of eccentric force

Causes rotation (angular motion)

Force through centre of mass

Produces linear motion only, no rotation

Torque

The turning effect of a force

Distance from centre of mass

Increasing distance increases torque and angular motion

Sporting example of angular motion

A diver performing a somersault

Off-centre hit in hockey

An eccentric force creates torque, causing the ball to spin

Greatest angular motion

Force applied farthest from the axis of rotation

Angular momentum

A measure of rotational motion

Factors affecting angular momentum

Mass distribution and rotational speed

Conservation of angular momentum

Occurs when no external eccentric forces act

Gymnast rotation in air

Angular momentum is conserved

Skater pulling arms in

Increases rotational speed

Angular momentum during arm pull-in

Remains constant

When angular momentum is not conserved

When external forces act, such as pushing off the ground

Force that changes angular momentum

An external eccentric force

Hockey ball angular momentum conserved

After the ball leaves the stick

Angular motion vs angular momentum

Angular motion requires eccentric force; conservation requires no external torque