Assume that you are replacing the bearings in the wheel of your
bicycle. To test if you have done a good job, you hold the axle
vertically and give it a spin. The axle at the center of the wheel
is stationary. The wheel as a whole has no**
****translational motion**. But the wheel is turning around
the axle. Every point on the wheel undergoes circular motion about
an axis of rotation. Such motion around an axis of rotation is
called **rotational motion**.

Extended object can have translational and
rotational motion. Any motion of an extended object can be viewed
as a combination of translational motion of
the center of mass and rotational motion about the center of mass.

A foam square has a blue LED near its CM and a red LED near its edge. If we toss the sponge we can easily observe the parabolic motion of the CM and the rotational motion about the CM.

Assume you make a chalk mark on the rim of the wheel and you
originally orient the wheel so that the chalk mark is facing you.
This is your reference orientation. The
**angular position** of the wheel describes its orientation
relative to this reference orientation. As the wheel rotates, this
angular position is changing. The **angular
displacement **measures how far it has rotated from its
reference orientation. It is often
convenient to orient a coordinate system such that the z-axis
coincides with the axis of rotation and the x-axis defines the
reference orientation. Then the angular displacement
θ of
a point P on the wheel is the angle θ a line from the axis of rotation
to the point P makes with the x-axis.

Angles can be measured in units of degrees or
radians. 360 degrees = 2π radians. When describing rotational motion it is most
convenient to measure angles in units of radians.

To find out how fast the wheel is rotating, we measure its
**angular speed **ω.
The average angular speed is given by

ω_{avg} = (θ_{f} - θ_{i})/(t_{f}
- t_{i}) = Δθ/Δt.

Every point on the wheel has the same angular speed. The
units of angular speed are rad/s or s^{-1}, because
radians are not dimensional.

Assume that you turn the axle of the spinning wheel from
vertical to horizontal. The wheel is still spinning with the same
angular speed, but its **angular velocity**** ω** has changed. Angular
velocity is a has magnitude and direction.
Its magnitude is the angular speed, and its direction is the
direction of the axis of rotation. There is, however, a subtlety we
have to take care of. Assume that the axis of rotation is
vertical. What is the sense of rotation? Is the wheel spinning
clockwise or counterclockwise as viewed from above? Just saying the
axis is vertical does not tell us the sense
of rotation.

To specify the sense of rotation we use a convention called the
**right-hand rule**. If the fingers
of your right hand are curling to indicate which way the wheel is
turning, then the thumb of you right hand is pointing in the
direction of the axis of rotation. This is the direction of the
angular velocity.

The **angular acceleration**** α**
is defined as the rate of change of the
angular velocity. The average angular acceleration is given by

The angular velocity changes when the rotation rate is increasing or decreasing and when the axis of rotation changes direction. The angular acceleration is a vector.

What is the magnitude of the angular velocity,
**ω**, of the second hand of a clock?
What is the direction of **ω** as
you view a clock hanging vertically? What is the magnitude of
the angular acceleration of the second hand?

Solution:

The second hand of a clock goes through an angular
displacement of 2π in one
minute. Its angular speed is ω = 2π/(60 s) = 0.105/s. The direction
of **ω** is
perpendicular to the face of the clock, pointing into the
face of the clock. The average angular acceleration of
the second hand is zero.

What is the average angular speed, in radians per second, of

(a) the Earth in its orbit about the Sun, and

(b) the Moon in its orbit about the Earth?

Solution:

(a) The earth orbits the sun once every year. The
angular speed of the earth in its orbit about the sun is
ω = 2π/year = 2π/(365*24*60*60 s) = 2*10^{-7}/s.

(b) The moon orbits the earth once 27.3 days. The
angular speed of the moon in its orbit about the earth
is ω = 2π/(27.3
days) = 2π/(27.3*24*60*60 s) = 2.7*10^{-6}/s.