10 FixedAxis Rotation
10.1 Rotational Variables
Learning Objectives
By the end of this section, you will be able to:
 Describe the physical meaning of rotational variables as applied to fixedaxis rotation
 Explain how angular velocity is related to tangential speed
 Calculate the instantaneous angular velocity given the angular position function
 Find the angular velocity and angular acceleration in a rotating system
 Calculate the average angular acceleration when the angular velocity is changing
 Calculate the instantaneous angular acceleration given the angular velocity function
So far in this text, we have mainly studied translational motion, including the variables that describe it: displacement, velocity, and acceleration. Now we expand our description of motion to rotation—specifically, rotational motion about a fixed axis. We will find that rotational motion is described by a set of related variables similar to those we used in translational motion.
Angular Velocity
Uniform circular motion (discussed previously in Motion in Two and Three Dimensions) is motion in a circle at constant speed. Although this is the simplest case of rotational motion, it is very useful for many situations, and we use it here to introduce rotational variables.
In (Figure), we show a particle moving in a circle. The coordinate system is fixed and serves as a frame of reference to define the particle’s position. Its position vector from the origin of the circle to the particle sweeps out the angle
, which increases in the counterclockwise direction as the particle moves along its circular path. The angle
is called the angular position of the particle. As the particle moves in its circular path, it also traces an arc length s.
The angle is related to the radius of the circle and the arc length by
The angle
, the angular position of the particle along its path, has units of radians (rad). There are
radians in
Note that the radian measure is a ratio of length measurements, and therefore is a dimensionless quantity. As the particle moves along its circular path, its angular position changes and it undergoes angular displacements
We can assign vectors to the quantities in (Figure). The angle
is a vector out of the page in (Figure). The angular position vector
and the arc length
both lie in the plane of the page. These three vectors are related to each other by
That is, the arc length is the cross product of the angle vector and the position vector, as shown in (Figure).
The magnitude of the angular velocity, denoted by
, is the time rate of change of the angle
as the particle moves in its circular path. The instantaneous angular velocity is defined as the limit in which
in the average angular velocity
:
where
is the angle of rotation ((Figure)). The units of angular velocity are radians per second (rad/s). Angular velocity can also be referred to as the rotation rate in radians per second. In many situations, we are given the rotation rate in revolutions/s or cycles/s. To find the angular velocity, we must multiply revolutions/s by
, since there are
radians in one complete revolution. Since the direction of a positive angle in a circle is counterclockwise, we take counterclockwise rotations as being positive and clockwise rotations as negative.
We can see how angular velocity is related to the tangential speed of the particle by differentiating (Figure) with respect to time. We rewrite (Figure) as
Taking the derivative with respect to time and noting that the radius r is a constant, we have
where
. Here
is just the tangential speed of the particle in (Figure). Thus, by using (Figure), we arrive at
That is, the tangential speed of the particle is its angular velocity times the radius of the circle. From (Figure), we see that the tangential speed of the particle increases with its distance from the axis of rotation for a constant angular velocity. This effect is shown in (Figure). Two particles are placed at different radii on a rotating disk with a constant angular velocity. As the disk rotates, the tangential speed increases linearly with the radius from the axis of rotation. In (Figure), we see that
and
. But the disk has a constant angular velocity, so
. This means
or
. Thus, since
,
.
Up until now, we have discussed the magnitude of the angular velocity
which is a scalar quantity—the change in angular position with respect to time. The vector
is the vector associated with the angular velocity and points along the axis of rotation. This is useful because when a rigid body is rotating, we want to know both the axis of rotation and the direction that the body is rotating about the axis, clockwise or counterclockwise. The angular velocity
gives us this information. The angular velocity
has a direction determined by what is called the righthand rule. The righthand rule is such that if the fingers of your right hand wrap counterclockwise from the xaxis (the direction in which
increases) toward the yaxis, your thumb points in the direction of the positive zaxis ((Figure)). An angular velocity
that points along the positive zaxis therefore corresponds to a counterclockwise rotation, whereas an angular velocity
that points along the negative zaxis corresponds to a clockwise rotation.
We can verify the righthandrule using the vector expression for the arc length
, (Figure). If we differentiate this equation with respect to time, we find
Since
is constant, the term
. Since
is the tangential velocity and
is the angular velocity, we have
That is, the tangential velocity is the cross product of the angular velocity and the position vector, as shown in (Figure). From part (a) of this figure, we see that with the angular velocity in the positive zdirection, the rotation in the xyplane is counterclockwise. In part (b), the angular velocity is in the negative zdirection, giving a clockwise rotation in the xyplane.
Example
Rotation of a Flywheel
A flywheel rotates such that it sweeps out an angle at the rate of
radians. The wheel rotates counterclockwise when viewed in the plane of the page. (a) What is the angular velocity of the flywheel? (b) What direction is the angular velocity? (c) How many radians does the flywheel rotate through in 30 s? (d) What is the tangential speed of a point on the flywheel 10 cm from the axis of rotation?
Strategy
The functional form of the angular position of the flywheel is given in the problem as
, so by taking the derivative with respect to time, we can find the angular velocity. We use the righthand rule to find the angular velocity. To find the angular displacement of the flywheel during 30 s, we seek the angular displacement
, where the change in angular position is between 0 and 30 s. To find the tangential speed of a point at a distance from the axis of rotation, we multiply its distance times the angular velocity of the flywheel.
Solution

. We see that the angular velocity is a constant.
 By the righthand rule, we curl the fingers in the direction of rotation, which is counterclockwise in the plane of the page, and the thumb points in the direction of the angular velocity, which is out of the page.

.

.
Significance
In 30 s, the flywheel has rotated through quite a number of revolutions, about 215 if we divide the angular displacement by
. A massive flywheel can be used to store energy in this way, if the losses due to friction are minimal. Recent research has considered superconducting bearings on which the flywheel rests, with zero energy loss due to friction.
Angular Acceleration
We have just discussed angular velocity for uniform circular motion, but not all motion is uniform. Envision an ice skater spinning with his arms outstretched—when he pulls his arms inward, his angular velocity increases. Or think about a computer’s hard disk slowing to a halt as the angular velocity decreases. We will explore these situations later, but we can already see a need to define an angular acceleration for describing situations where
changes. The faster the change in
, the greater the angular acceleration. We define the instantaneous angular acceleration
as the derivative of angular velocity with respect to time:
where we have taken the limit of the average angular acceleration,
as
.
The units of angular acceleration are (rad/s)/s, or
.
In the same way as we defined the vector associated with angular velocity
, we can define
, the vector associated with angular acceleration ((Figure)). If the angular velocity is along the positive zaxis, as in (Figure), and
is positive, then the angular acceleration
is positive and points along the
axis. Similarly, if the angular velocity
is along the positive zaxis and
is negative, then the angular acceleration is negative and points along the
axis.
We can express the tangential acceleration vector as a cross product of the angular acceleration and the position vector. This expression can be found by taking the time derivative of
and is left as an exercise:
The vector relationships for the angular acceleration and tangential acceleration are shown in (Figure).
We can relate the tangential acceleration of a point on a rotating body at a distance from the axis of rotation in the same way that we related the tangential speed to the angular velocity. If we differentiate (Figure) with respect to time, noting that the radius r is constant, we obtain
Thus, the tangential acceleration
is the radius times the angular acceleration. (Figure) and (Figure) are important for the discussion of rolling motion (see Angular Momentum).
Let’s apply these ideas to the analysis of a few simple fixedaxis rotation scenarios. Before doing so, we present a problemsolving strategy that can be applied to rotational kinematics: the description of rotational motion.
ProblemSolving Strategy: Rotational Kinematics
 Examine the situation to determine that rotational kinematics (rotational motion) is involved.
 Identify exactly what needs to be determined in the problem (identify the unknowns). A sketch of the situation is useful.
 Make a complete list of what is given or can be inferred from the problem as stated (identify the knowns).
 Solve the appropriate equation or equations for the quantity to be determined (the unknown). It can be useful to think in terms of a translational analog, because by now you are familiar with the equations of translational motion.
 Substitute the known values along with their units into the appropriate equation and obtain numerical solutions complete with units. Be sure to use units of radians for angles.
 Check your answer to see if it is reasonable: Does your answer make sense?
Now let’s apply this problemsolving strategy to a few specific examples.
Example
A Spinning Bicycle Wheel
A bicycle mechanic mounts a bicycle on the repair stand and starts the rear wheel spinning from rest to a final angular velocity of 250 rpm in 5.00 s. (a) Calculate the average angular acceleration in
. (b) If she now hits the brakes, causing an angular acceleration of −87.3
, how long does it take the wheel to stop?
Strategy
The average angular acceleration can be found directly from its definition
because the final angular velocity and time are given. We see that
and
is 5.00 s. For part (b), we know the angular acceleration and the initial angular velocity. We can find the stopping time by using the definition of average angular acceleration and solving for
, yielding
Solution
 Entering known information into the definition of angular acceleration, we get
Because
is in revolutions per minute (rpm) and we want the standard units of
for angular acceleration, we need to convert from rpm to rad/s:
Entering this quantity into the expression for
, we get
 Here the angular velocity decreases from 26.2 rad/s (250 rpm) to zero, so that
is −26.2 rad/s, and
is given to be –87.3
. Thus,
Significance
Note that the angular acceleration as the mechanic spins the wheel is small and positive; it takes 5 s to produce an appreciable angular velocity. When she hits the brake, the angular acceleration is large and negative. The angular velocity quickly goes to zero.
Check Your Understanding
The fan blades on a turbofan jet engine (shown below) accelerate from rest up to a rotation rate of 40.0 rev/s in 20 s. The increase in angular velocity of the fan is constant in time. (The GE90110B1 turbofan engine mounted on a Boeing 777, as shown, is currently the largest turbofan engine in the world, capable of thrusts of 330–510 kN.)
(a) What is the average angular acceleration?
(b) What is the instantaneous angular acceleration at any time during the first 20 s?
[revealanswer q=”821157″]Show Answer[/revealanswer]
[hiddenanswer a=”821157″]a.
,
; b. Since the angular velocity increases linearly, there has to be a constant acceleration throughout the indicated time. Therefore, the instantaneous angular acceleration at any time is the solution to
.[/hiddenanswer]
Example
Wind Turbine
A wind turbine ((Figure)) in a wind farm is being shut down for maintenance. It takes 30 s for the turbine to go from its operating angular velocity to a complete stop in which the angular velocity function is
. If the turbine is rotating counterclockwise looking into the page, (a) what are the directions of the angular velocity and acceleration vectors? (b) What is the average angular acceleration? (c) What is the instantaneous angular acceleration at
Strategy
 We are given the rotational sense of the turbine, which is counterclockwise in the plane of the page. Using the right hand rule ((Figure)), we can establish the directions of the angular velocity and acceleration vectors.
 We calculate the initial and final angular velocities to get the average angular acceleration. We establish the sign of the angular acceleration from the results in (a).
 We are given the functional form of the angular velocity, so we can find the functional form of the angular acceleration function by taking its derivative with respect to time.
Solution
 Since the turbine is rotating counterclockwise, angular velocity
points out of the page. But since the angular velocity is decreasing, the angular acceleration
points into the page, in the opposite sense to the angular velocity.
 The initial angular velocity of the turbine, setting
. The final angular velocity is zero, so the average angular acceleration is
 Taking the derivative of the angular velocity with respect to time gives
Significance
We found from the calculations in (a) and (b) that the angular acceleration
and the average angular acceleration
are negative. The turbine has an angular acceleration in the opposite sense to its angular velocity.
We now have a basic vocabulary for discussing fixedaxis rotational kinematics and relationships between rotational variables. We discuss more definitions and connections in the next section.
Summary
 The angular position
of a rotating body is the angle the body has rotated through in a fixed coordinate system, which serves as a frame of reference.
 The angular velocity of a rotating body about a fixed axis is defined as
, the rotational rate of the body in radians per second. The instantaneous angular velocity of a rotating body
is the derivative with respect to time of the angular position
, found by taking the limit
in the average angular velocity
. The angular velocity relates
to the tangential speed of a point on the rotating body through the relation
, where r is the radius to the point and
is the tangential speed at the given point.
 The angular velocity
is found using the righthand rule. If the fingers curl in the direction of rotation about a fixed axis, the thumb points in the direction of
(see (Figure)).
 If the system’s angular velocity is not constant, then the system has an angular acceleration. The average angular acceleration over a given time interval is the change in angular velocity over this time interval,
. The instantaneous angular acceleration is the time derivative of angular velocity,
. The angular acceleration
is found by locating the angular velocity. If a rotation rate of a rotating body is decreasing, the angular acceleration is in the opposite direction to
. If the rotation rate is increasing, the angular acceleration is in the same direction as
.
 The tangential acceleration of a point at a radius from the axis of rotation is the angular acceleration times the radius to the point.
Conceptual Questions
A clock is mounted on the wall. As you look at it, what is the direction of the angular velocity vector of the second hand?
[revealanswer q=”fsid1167134656138″]Show Solution[/revealanswer]
[hiddenanswer a=”fsid1167134656138″]
The second hand rotates clockwise, so by the righthand rule, the angular velocity vector is into the wall.
[/hiddenanswer]
What is the value of the angular acceleration of the second hand of the clock on the wall?
A baseball bat is swung. Do all points on the bat have the same angular velocity? The same tangential speed?
[revealanswer q=”fsid1167134532422″]Show Solution[/revealanswer]
[hiddenanswer a=”fsid1167134532422″]
They have the same angular velocity. Points further out on the bat have greater tangential speeds.
[/hiddenanswer]
The blades of a blender on a counter are rotating clockwise as you look into it from the top. If the blender is put to a greater speed what direction is the angular acceleration of the blades?
Problems
Calculate the angular velocity of Earth.
A track star runs a 400m race on a 400m circular track in 45 s. What is his angular velocity assuming a constant speed?
[revealanswer q=”fsid1167134645402″]Show Solution[/revealanswer]
[hiddenanswer a=”fsid1167134645402″]
[/hiddenanswer]
A wheel rotates at a constant rate of
. (a) What is its angular velocity in radians per second? (b) Through what angle does it turn in 10 s? Express the solution in radians and degrees.
A particle moves 3.0 m along a circle of radius 1.5 m. (a) Through what angle does it rotate? (b) If the particle makes this trip in 1.0 s at a constant speed, what is its angular velocity? (c) What is its acceleration?
[revealanswer q=”fsid1167134950880″]Show Solution[/revealanswer]
[hiddenanswer a=”fsid1167134950880″]
a.
; b.
; c.
[/hiddenanswer]
A compact disc rotates at 500 rev/min. If the diameter of the disc is 120 mm, (a) what is the tangential speed of a point at the edge of the disc? (b) At a point halfway to the center of the disc?
Unreasonable results. The propeller of an aircraft is spinning at 10 rev/s when the pilot shuts off the engine. The propeller reduces its angular velocity at a constant
for a time period of 40 s. What is the rotation rate of the propeller in 40 s? Is this a reasonable situation?
[revealanswer q=”fsid1167134534735″]Show Solution[/revealanswer]
[hiddenanswer a=”fsid1167134534735″]
The propeller takes only
to come to rest, when the propeller is at 0 rad/s, it would start rotating in the opposite direction. This would be impossible due to the magnitude of forces involved in getting the propeller to stop and start rotating in the opposite direction.
[/hiddenanswer]
A gyroscope slows from an initial rate of 32.0 rad/s at a rate of
. How long does it take to come to rest?
On takeoff, the propellers on a UAV (unmanned aerial vehicle) increase their angular velocity from rest at a rate of
for 3.0 s. (a) What is the instantaneous angular velocity of the propellers at
? (b) What is the angular acceleration?
[revealanswer q=”fsid1167134542349″]Show Solution[/revealanswer]
[hiddenanswer a=”fsid1167134542349″]
a.
; b.
[/hiddenanswer]
The angular position of a rod varies as
radians from time
. The rod has two beads on it as shown in the following figure, one at 10 cm from the rotation axis and the other at 20 cm from the rotation axis. (a) What is the instantaneous angular velocity of the rod at
(b) What is the angular acceleration of the rod? (c) What are the tangential speeds of the beads at
(d) What are the tangential accelerations of the beads at
(e) What are the centripetal accelerations of the beads at
Glossary
 angular acceleration
 time rate of change of angular velocity
 angular position
 angle a body has rotated through in a fixed coordinate system
 angular velocity
 time rate of change of angular position
 instantaneous angular acceleration
 derivative of angular velocity with respect to time
 instantaneous angular velocity
 derivative of angular position with respect to time