← Describing rotation

Angular displacement

Dagny rides a Ferris wheel. Her seat goes a quarter of the way around the wheel.

How far around is that, exactly?

Earlier in the course you saw displacement: how far an object's position changed.

Angular displacement is the same idea for turning: how far around the object has swept.

Draw the line from the wheel's center to Dagny's seat.

As the wheel turns, that line sweeps out an angle.

90°the wheel's centerthe seat, at the starta quarter of the way around
A quarter of the way around: the center-to-seat line sweeps 90°.
The angle swept out by the line from the circle's center to the moving object is called the object's angular displacement.

A quarter of the way around sweeps 90°.

Half of the way around sweeps 180°, and the whole way around sweeps 360°.

180°the wheel's centerthe seat, at the starthalf of the way around
Half of the way around sweeps 180°.
360°the wheel's centerthe whole way around:back at the start
The whole way around sweeps 360°.

A merry-go-round carries a rider three quarters of the way around. Enter the rider's angular displacement in degrees.

Coral says: 'A rider who goes all the way around once sweeps an angular displacement of 360°.' Is Coral correct?

The angle measures the turn itself. It does not depend on how big the circle is.

A merry-go-round carries a rider half of the way around. State the rider's angular displacement.

The line from the center to the rider sweeps half of a full turn.

Half of 360° is 180°.

The rider's angular displacement is 180°.

A ceiling fan blade turns exactly once around. State the blade's angular displacement.

The line from the hub to the blade tip sweeps one full turn.

One full turn is 360°.

The blade's angular displacement is 360°.

Your turn

A Ferris wheel seat goes one and a half times around the wheel. Enter the seat's total angular displacement in degrees.

A cart rides a circular track. Which angle is the cart's angular displacement?

Two riders sit on one carousel, one 2 m from the center and one 4 m out. The carousel turns a third of the way around. Which statement is correct?

The angle swept out by the line from the circle's center to a moving object is called the object's ___.