12: Angular Momentum


Angular momentum is the rotational counterpart of linear momentum. Any massive object that rotates about an axis carries angular momentum, including rotating flywheels, planets, stars, hurricanes, tornadoes, whirlpools, and so on. The concept of conservation of angular momentum is discussed later in this section. In the main part of this section, we explore the intricacies of angular momentum of rigid bodies such as a top, and also of point particles and systems of particles. But to be complete, we start with a discussion of rolling motion, which builds upon the concepts of the previous section.

• 12.1: Prelude to Angular Momentum
A helicopter can be used to illustrate the concept of angular momentum. The lift blades spin about a vertical axis through the main body and carry angular momentum. The body of the helicopter tends to rotate in the opposite sense in order to conserve angular momentum. The small rotors at the tail of the aircraft provide a counter thrust against the body to prevent this from happening, and the helicopter stabilizes itself.
• 12.2: Rolling Motion
In rolling motion without slipping, a static friction force is present between the rolling object and the surface. The linear velocity, acceleration, and distance of the center of mass are the angular variables multiplied by the radius of the object. In rolling motion with slipping, a kinetic friction force arises between the rolling object and the surface. Energy conservation can be used to analyze rolling motion since energy is conserved in rolling motion without slipping.
• 12.3: Angular Momentum
The angular momentum of a single particle about a designated origin is the vector product of the position vector in the given coordinate system and the particle’s linear momentum. The net torque on a system about a given origin is the time derivative of the angular momentum about that origin. A rigid rotating body has angular momentum directed along the axis of rotation.
• 12.4: Conservation of Angular Momentum
In the absence of external torques, a system’s total angular momentum is conserved. The angular velocity is inversely proportional to the moment of inertia, so if the moment of inertia decreases, the angular velocity must increase to conserve angular momentum. Systems containing both point particles and rigid bodies can be analyzed using conservation of angular momentum. The angular momentum of all bodies in the system must be taken about a common axis.
• 12.5: Precession of a Gyroscope
When a gyroscope is set on a pivot near the surface of Earth, it precesses around a vertical axis, since the torque is always horizontal and perpendicular to the angular momentum vector. If the gyroscope is not spinning, it acquires angular momentum in the direction of the torque, and it rotates about a horizontal axis, falling over just as we would expect.
• 12.E: Angular Momentum (Exercises)
• 12.S: Angular Momentum (Summary)

Thumbnail: A gyroscope is a device used for measuring or maintaining orientation and angular velocity. It is a spinning wheel or disc in which the axis of rotation (spin axis) is free to assume any orientation by itself. When rotating, the orientation of this axis is unaffected by tilting or rotation of the mounting, according to the conservation of angular momentum. (Public Domain; LucasVB).

​​​​​Contributors

• Samuel J. Ling (Truman State University), Jeff Sanny (Loyola Marymount University), and Bill Moebs with many contributing authors. This work is licensed by OpenStax University Physics under a Creative Commons Attribution License (by 4.0).

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