Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
College Physics 2e

10.5 Angular Momentum and Its Conservation

College Physics 2e10.5 Angular Momentum and Its Conservation

Learning Objectives

By the end of this section, you will be able to:

  • Understand the analogy between angular momentum and linear momentum.
  • Observe the relationship between torque and angular momentum.
  • Apply the law of conservation of angular momentum.

Why does Earth keep on spinning? What started it spinning to begin with? And how does an ice skater manage to spin faster and faster simply by pulling her arms in? Why does she not have to exert a torque to spin faster? Questions like these have answers based in angular momentum, the rotational analog to linear momentum.

By now the pattern is clear—every rotational phenomenon has a direct translational analog. It seems quite reasonable, then, to define angular momentum LL as


This equation is an analog to the definition of linear momentum as p=mvp=mv. Units for linear momentum are kgm/skgm/s while units for angular momentum are kgm2/skgm2/s. As we would expect, an object that has a large moment of inertia II, such as Earth, has a very large angular momentum. An object that has a large angular velocity ωω, such as a centrifuge, also has a rather large angular momentum.

Making Connections

Angular momentum is completely analogous to linear momentum, first presented in Uniform Circular Motion and Gravitation. It has the same implications in terms of carrying rotation forward, and it is conserved when the net external torque is zero. Angular momentum, like linear momentum, is also a property of the atoms and subatomic particles.

Example 10.11

Calculating Angular Momentum of the Earth


No information is given in the statement of the problem; so we must look up pertinent data before we can calculate L=L=. First, according to Figure 10.11, the formula for the moment of inertia of a sphere is

I = 2 MR 2 5 I = 2 MR 2 5

so that

L = = 2 MR 2 ω 5 . L = = 2 MR 2 ω 5 .

Earth’s mass MM is 5.979×1024 kg5.979×1024 kg and its radius RR is 6.376×106m6.376×106m. The Earth’s angular velocity ωω is, of course, exactly one revolution per day, but we must covert ωω to radians per second to do the calculation in SI units.


Substituting known information into the expression for LL and converting ωω to radians per second gives

L = 0.45.979×1024kg6.376×106 m21revd = 9.72×1037kgm2rev/d.L = 0.45.979×1024kg6.376×106 m21revd = 9.72×1037kgm2rev/d.

Substituting rad for 11 rev and 8.64×104s8.64×104s for 1 day gives

L = 9.72×1037kgm2 rad/rev8.64×104 s/d1 rev/d = 7.07×1033 kgm2/s. L = 9.72×1037kgm2 rad/rev8.64×104 s/d1 rev/d = 7.07×1033 kgm2/s.


This number is large, demonstrating that Earth, as expected, has a tremendous angular momentum. The answer is approximate, because we have assumed a constant density for Earth in order to estimate its moment of inertia.

When you push a merry-go-round, spin a bike wheel, or open a door, you exert a torque. If the torque you exert is greater than opposing torques, then the rotation accelerates, and angular momentum increases. The greater the net torque, the more rapid the increase in LL. The relationship between torque and angular momentum is

net τ=ΔLΔ τ=ΔLΔt.

This expression is exactly analogous to the relationship between force and linear momentum, F=Δp/ΔtF=Δp/Δt. The equation net τ=ΔLΔtnet τ=ΔLΔt is very fundamental and broadly applicable. It is, in fact, the rotational form of Newton’s second law.

Example 10.12

Calculating the Torque Putting Angular Momentum Into a Rotating Food Tray

Figure 10.19 shows a rotating food tray, often called a lazy Susan, being turned by a person in quest of sustenance. Suppose the person exerts a 2.50 N force perpendicular to the lazy Susan’s 0.260-m radius for 0.150 s. (a) What is the final angular momentum of the lazy Susan if it starts from rest, assuming friction is negligible? (b) What is the final angular velocity of the lazy Susan, given that its mass is 4.00 kg and assuming its moment of inertia is that of a disk?

The given figure shows a lazy Susan on which various eatables like cake, salad grapes, and a drink are kept. A hand is shown that applies a force F, indicated by a leftward pointing horizontal arrow. This force is perpendicular to the radius r and thus tangential to the circular lazy Susan.
Figure 10.19 A partygoer exerts a torque on a lazy Susan to make it rotate. The equation net τ=ΔLΔtnet τ=ΔLΔt gives the relationship between torque and the angular momentum produced.


We can find the angular momentum by solving net τ= Δ L Δ t net τ= Δ L Δ t for ΔLΔL, and using the given information to calculate the torque. The final angular momentum equals the change in angular momentum, because the lazy Susan starts from rest. That is, ΔL=LΔL=L. To find the final velocity, we must calculate ωω from the definition of LL in L=L=.

Solution for (a)

Solving net τ=ΔLΔtnet τ=ΔLΔt for ΔLΔL gives

ΔL=net τΔt.ΔL=net τΔt.

Because the force is perpendicular to rr, we see that net τ=rFnet τ=rF, so that

L = rFΔt=(0.260 m)(2.50 N)(0.150 s) = 9.75×102kgm2/s.L = rFΔt=(0.260 m)(2.50 N)(0.150 s) = 9.75×102kgm2/s.

Solution for (b)

The final angular velocity can be calculated from the definition of angular momentum,


Solving for ωω and substituting the formula for the moment of inertia of a disk into the resulting equation gives


And substituting known values into the preceding equation yields

ω=9.75×102 kgm2/s0.5004.00 kg0.260 m2=0.721 rad/s.ω=9.75×102 kgm2/s0.5004.00 kg0.260 m2=0.721 rad/s.


Note that the imparted angular momentum does not depend on any property of the object but only on torque and time. The final angular velocity is equivalent to one revolution in 8.71 s (determination of the time period is left as an exercise for the reader), which is about right for a lazy Susan.

Example 10.13

Calculating the Torque in a Kick

The person whose leg is shown in Figure 10.20 kicks his leg by exerting a 2000-N force with his upper leg muscle. The effective perpendicular lever arm is 2.20 cm. Given the moment of inertia of the lower leg is 1.25 kgm21.25 kgm2, (a) find the angular acceleration of the leg. (b) Neglecting the gravitational force, what is the rotational kinetic energy of the leg after it has rotated through 57.57. (1.00 rad)?

The figure shows a human leg, from the thighs to the feet which is bent at the knee joint. The radius of curvature of the knee is indicated as r equal to two point two zero centimeters and the moment of inertia of the lower half of the leg is indicated as I equal to one point two five kilogram meter square. The direction of torque is indicated by a red arrow in anti-clockwise direction, near the knee.
Figure 10.20 The muscle in the upper leg gives the lower leg an angular acceleration and imparts rotational kinetic energy to it by exerting a torque about the knee. FF is a vector that is perpendicular to rr. This example examines the situation.


The angular acceleration can be found using the rotational analog to Newton’s second law, or α=net τ/Iα=net τ/I. The moment of inertia II is given and the torque can be found easily from the given force and perpendicular lever arm. Once the angular acceleration αα is known, the final angular velocity and rotational kinetic energy can be calculated.

Solution to (a)

From the rotational analog to Newton’s second law, the angular acceleration αα is

α=net τI.α=net τI.

Because the force and the perpendicular lever arm are given and the leg is vertical so that its weight does not create a torque, the net torque is thus

net τ = rF = 0.0220 m2000 N = 44.0 Nm. net τ = rF = 0.0220 m2000 N = 44.0 Nm.

Substituting this value for the torque and the given value for the moment of inertia into the expression for αα gives

α=44.0 Nm1.25 kgm2=35.2 rad/s2.α=44.0 Nm1.25 kgm2=35.2 rad/s2.

Solution to (b)

The final angular velocity can be calculated from the kinematic expression

ω2= ω02+2αθω2= ω02+2αθ


ω 2 = 2 αθ ω 2 = 2 αθ

because the initial angular velocity is zero. The kinetic energy of rotation is

KE rot = 1 2 2 KE rot = 1 2 2

so it is most convenient to use the value of ω2ω2 just found and the given value for the moment of inertia. The kinetic energy is then

KErot = 0.51.25 kgm270.4 rad2/s2 = 44.0 J. KErot = 0.51.25 kgm270.4 rad2/s2 = 44.0 J.


These values are reasonable for a person kicking his leg starting from the position shown. The weight of the leg can be neglected in part (a) because it exerts no torque when the center of gravity of the lower leg is directly beneath the pivot in the knee. In part (b), the force exerted by the upper leg is so large that its torque is much greater than that created by the weight of the lower leg as it rotates. The rotational kinetic energy given to the lower leg is enough that it could give a ball a significant velocity by transferring some of this energy in a kick.

Making Connections: Conservation Laws

Angular momentum, like energy and linear momentum, is conserved. This universally applicable law is another sign of underlying unity in physical laws. Angular momentum is conserved when net external torque is zero, just as linear momentum is conserved when the net external force is zero.

Conservation of Angular Momentum

We can now understand why Earth keeps on spinning. As we saw in the previous example, ΔL=(netτ)ΔtΔL=(netτ)Δt. This equation means that, to change angular momentum, a torque must act over some period of time. Because Earth has a large angular momentum, a large torque acting over a long time is needed to change its rate of spin. So what external torques are there? Tidal friction exerts torque that is slowing Earth’s rotation, but tens of millions of years must pass before the change is very significant. Recent research indicates the length of the day was 18 h some 900 million years ago. Only the tides exert significant retarding torques on Earth, and so it will continue to spin, although ever more slowly, for many billions of years.

What we have here is, in fact, another conservation law. If the net torque is zero, then angular momentum is constant or conserved. We can see this rigorously by considering net τ= Δ L Δ t net τ= Δ L Δ t for the situation in which the net torque is zero. In that case,

net τ=0net τ=0

implying that

Δ L Δ t =0. Δ L Δ t =0.

If the change in angular momentum ΔLΔL is zero, then the angular momentum is constant; thus,

L = constant net τ = 0 L = constant net τ = 0


L = L net τ = 0 . L = L net τ = 0 .

These expressions are the law of conservation of angular momentum. Conservation laws are as scarce as they are important.

An example of conservation of angular momentum is seen in Figure 10.21, in which an ice skater is executing a spin. The net torque on her is very close to zero, because there is relatively little friction between her skates and the ice and because the friction is exerted very close to the pivot point. (Both FF and rr are small, and so ττ is negligibly small.) Consequently, she can spin for quite some time. She can do something else, too. She can increase her rate of spin by pulling her arms and legs in. Why does pulling her arms and legs in increase her rate of spin? The answer is that her angular momentum is constant, so that


Expressing this equation in terms of the moment of inertia,


where the primed quantities refer to conditions after she has pulled in her arms and reduced her moment of inertia. Because II is smaller, the angular velocity ωω must increase to keep the angular momentum constant. The change can be dramatic, as the following example shows.

The image a shows an ice skater spinning on the tip of her skate with both her arms and one leg extended. The image b shows the ice skater spinning on the tip of one skate, with her arms crossed and one leg supported on another.
Figure 10.21 (a) An ice skater is spinning on the tip of her skate with her arms extended. Her angular momentum is conserved because the net torque on her is negligibly small. In the next image, her rate of spin increases greatly when she pulls in her arms, decreasing her moment of inertia. The work she does to pull in her arms results in an increase in rotational kinetic energy.

Example 10.14

Calculating the Angular Momentum of a Spinning Skater

Suppose an ice skater, such as the one in Figure 10.21, is spinning at 0.800 rev/ s with her arms extended. She has a moment of inertia of 2.34kgm22.34kgm2 with her arms extended and of 0.363kgm20.363kgm2with her arms close to her body. (These moments of inertia are based on reasonable assumptions about a 60.0-kg skater.) (a) What is her angular velocity in revolutions per second after she pulls in her arms? (b) What is her rotational kinetic energy before and after she does this?


In the first part of the problem, we are looking for the skater’s angular velocity ωω after she has pulled in her arms. To find this quantity, we use the conservation of angular momentum and note that the moments of inertia and initial angular velocity are given. To find the initial and final kinetic energies, we use the definition of rotational kinetic energy given by


Solution for (a)

Because torque is negligible (as discussed above), the conservation of angular momentum given in =Iω=Iω is applicable. Thus,

L = L L = L


= I ω = I ω

Solving for ωωand substituting known values into the resulting equation gives

ω = I I ω = 2.34 kg m 2 0 .363 kg m 2 0.800 rev/s = 5.16 rev/s. ω = I I ω = 2.34 kg m 2 0 .363 kg m 2 0.800 rev/s = 5.16 rev/s.

Solution for (b)

Rotational kinetic energy is given by


The initial value is found by substituting known values into the equation and converting the angular velocity to rad/s:

KE rot = ( 0 . 5) 2 . 34 kg m 2 0 . 800 rev/s rad/rev 2 = 29.6 J. KE rot = ( 0 . 5) 2 . 34 kg m 2 0 . 800 rev/s rad/rev 2 = 29.6 J.

The final rotational kinetic energy is


Substituting known values into this equation gives

K E rot = 0 . 5 0 .363 kg m 2 5 . 16 rev/s 2π rad/rev 2 = 191 J. K E rot = 0 . 5 0 .363 kg m 2 5 . 16 rev/s 2π rad/rev 2 = 191 J.


In both parts, there is an impressive increase. First, the final angular velocity is large, although most world-class skaters can achieve spin rates about this great. Second, the final kinetic energy is much greater than the initial kinetic energy. The increase in rotational kinetic energy comes from work done by the skater in pulling in her arms. This work is internal work that depletes some of the skater’s food energy.

There are several other examples of objects that increase their rate of spin because something reduced their moment of inertia. Tornadoes are one example. Storm systems that create tornadoes are slowly rotating. When the radius of rotation narrows, even in a local region, angular velocity increases, sometimes to the furious level of a tornado. Earth is another example. Our planet was born from a huge cloud of gas and dust, the rotation of which came from turbulence in an even larger cloud. Gravitational forces caused the cloud to contract, and the rotation rate increased as a result. (See Figure 10.22.)

The first figure shows a cloud of dust and gas,which is in the shape of a distorted circle, rotating in anti-clockwise direction with an angular velocity omega, indicated by a curved black arrow, and having an angular momentum L. The second figure shows an elliptical cloud of dust with the Sun in the middle of it, rotating in anti-clockwise direction with an angular velocity omega dash, indicated by a curved black arrow, and having an angular momentum L. The third figure depicts the Solar System, with the Sun in the middle of it and the various planets revolve around it in their respective elliptical orbits in anti-clockwise direction, which is indicated by arrows. The angular momentum remains L.
Figure 10.22 The Solar System coalesced from a cloud of gas and dust that was originally rotating. The orbital motions and spins of the planets are in the same direction as the original spin and conserve the angular momentum of the parent cloud.

In case of human motion, one would not expect angular momentum to be conserved when a body interacts with the environment as its foot pushes off the ground. Astronauts floating in space aboard the International Space Station have no angular momentum relative to the inside of the ship if they are motionless. Their bodies will continue to have this zero value no matter how they twist about as long as they do not give themselves a push off the side of the vessel.

Check Your Understanding

Is angular momentum completely analogous to linear momentum? What, if any, are their differences?

Order a print copy

As an Amazon Associate we earn from qualifying purchases.


This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Attribution information Citation information

© Jan 19, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.