Learning Objectives
By the end of this section, you will be able to do the following:
- 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
The information presented in this section supports the following AP® learning objectives and science practices:
- 4.D.2.1 The student is able to describe a model of a rotational system and use that model to analyze a situation in which angular momentum changes due to interaction with other objects or systems. (S.P. 1.2, 1.4)
- 4.D.2.2 The student is able to plan a data collection and analysis strategy to determine the change in angular momentum of a system and relate it to interactions with other objects and systems. (S.P. 2.2)
- 4.D.3.1 The student is able to use appropriate mathematical routines to calculate values for initial or final angular momentum, or change in angular momentum of a system, or average torque or time during which the torque is exerted in analyzing a situation involving torque and angular momentum. (S.P. 2.2)
- 4.D.3.2 The student is able to plan a data collection strategy designed to test the relationship between the change in angular momentum of a system and the product of the average torque applied to the system and the time interval during which the torque is exerted. (S.P. 4.1, 4.2)
- 5.E.1.1 The student is able to make qualitative predictions about the angular momentum of a system for a situation in which there is no net external torque. (S.P. 6.4, 7.2)
- 5.E.1.2 The student is able to make calculations of quantities related to the angular momentum of a system when the net external torque on the system is zero. (S.P. 2.1, 2.2)
- 5.E.2.1 The student is able to describe or calculate the angular momentum and rotational inertia of a system in terms of the locations and velocities of objects that make up the system. Students are expected to do qualitative reasoning with compound objects. Students are expected to do calculations with a fixed set of extended objects and point masses. (S.P. 2.2)
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 L as
This equation is an analog to the definition of linear momentum as p=mv. Units for linear momentum are kg⋅m/s while units for angular momentum are kg⋅m2/s. As we would expect, an object that has a large moment of inertia I, such as Earth, has a very large angular momentum. An object that has a large angular velocity ω, 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
Strategy
No information is given in the statement of the problem; so we must look up pertinent data before we can calculate L=Iω. First, according to Figure 10.12, the formula for the moment of inertia of a sphere is
so that
Earth's mass M is 5.979×1024kg and its radius R is 6.376×106m. Earth's angular velocity ω is, of course, exactly one revolution per day, but we must covert ω to radians per second to do the calculation in SI units.
Solution
Substituting known information into the expression for L and converting ω to radians per second gives
Substituting 2π rad for 1 rev and 8.64×104s for 1 day gives
Discussion
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 L. The relationship between torque and angular momentum is
This expression is exactly analogous to the relationship between force and linear momentum, F=Δp/Δt. The equation netτ=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 Lazy Susan
Figure 10.21 shows a llazy susan food tray being rotated 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 kg and assuming its moment of inertia is that of a disk?
Strategy
We can find the angular momentum by solving netτ=ΔLΔt for ΔL, 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=L. To find the final velocity, we must calculate ω from the definition of L in L=Iω.
Solution for (a)
Solving netτ=ΔLΔt for ΔL gives
Because the force is perpendicular to r, we see that netτ=rF, so that
Solution for (b)
The final angular velocity can be calculated from the definition of angular momentum,
Solving for ω 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
Discussion
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.
Take-home Experiment
Plan an experiment to analyze changes to a system's angular momentum. Choose a system capable of rotational motion such as a lazy susan or a merry-go-round. Predict how the angular momentum of this system will change when you add an object to the lazy susan or jump onto the merry-go-round. What variables can you control? What are you measuring? In other words, what are your independent and dependent variables? Are there any independent variables that it would be useful to keep constant. Angular velocity, perhaps? Collect data in order to calculate or estimate the angular momentum of your system when in motion. What do you observe? Collect data in order to calculate the change in angular momentum as a result of the interaction you performed.
Using your data, how does the angular momentum vary with the size and location of an object added to the rotating system?
Example 10.13 Calculating the Torque in a Kick
The person whose leg is shown in Figure 10.22 kicks his leg by exerting a 2,000N 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 kg⋅m2, (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 57.3º or (1 rad)?
Strategy
The angular acceleration can be found using the rotational analog to Newton's second law, or α=netτ/I. The moment of inertia I is given and the torque can be found easily from the given force and perpendicular lever arm. Once the angular acceleration α 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 α is
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
Substituting this value for the torque and the given value for the moment of inertia into the expression for α gives
Solution to (b)
The final angular velocity can be calculated from the kinematic expression
or
because the initial angular velocity is zero. The kinetic energy of rotation is
so it is most convenient to use the value of ω2 just found and the given value for the moment of inertia. The kinetic energy is then
Discussion
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.