When Is Angular Momentum Not Conserved

Angular momentum is a fundamental concept in physics that describes the rotational motion of objects. That said, there are specific scenarios where angular momentum is not conserved. It is a vector quantity, meaning it has both magnitude and direction, and is conserved in isolated systems where no external torques act. Understanding these situations is crucial for analyzing rotational dynamics in various physical systems.

In classical mechanics, angular momentum is conserved when the net external torque acting on a system is zero. Because of that, this principle is analogous to the conservation of linear momentum in the absence of external forces. Still, when external torques are present, angular momentum can change over time.

$\tau_{\text{net}} = \frac{dL}{dt}$

where $\tau_{\text{net}}$ is the net external torque and $L$ is the angular momentum. If $\tau_{\text{net}} \neq 0$, then $L$ is not conserved.

One common scenario where angular momentum is not conserved is when an object interacts with its environment. Day to day, for example, a spinning ice skater can change their angular velocity by extending or retracting their arms. This change in angular velocity is due to the redistribution of mass, which alters the moment of inertia of the system. While the total angular momentum of the skater and the Earth (considered as an isolated system) is conserved, the skater's angular momentum alone is not conserved due to the external torque exerted by the ice.

Another situation where angular momentum is not conserved is in the presence of friction or other dissipative forces. When a rotating object experiences friction, such as a spinning top slowing down due to air resistance or contact with a surface, the angular momentum decreases over time. The energy lost due to friction is converted into heat, and the system is no longer isolated.

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In celestial mechanics, angular momentum is not conserved in systems where gravitational interactions are significant. Here's one way to look at it: in a binary star system, the stars orbit around their common center of mass. If one star loses mass through stellar winds or other processes, the angular momentum of the system changes. Similarly, in the formation of planetary systems, the conservation of angular momentum is key here, but it is not strictly conserved due to the complex interactions between forming planets, the protoplanetary disk, and the central star.

In quantum mechanics, angular momentum is quantized, meaning it can only take on specific discrete values. Even so, the conservation of angular momentum still holds in isolated quantum systems. In open quantum systems, where interactions with the environment are significant, angular momentum may not be conserved due to the exchange of angular momentum with the surroundings.

Good to know here that the conservation of angular momentum is a powerful tool for analyzing rotational motion, but it is not an absolute law. It is a consequence of the rotational symmetry of space, as described by Noether's theorem. When this symmetry is broken, such as in the presence of external torques or dissipative forces, angular momentum is not conserved.

At the end of the day, angular momentum is not conserved in situations where external torques act on a system, where dissipative forces are present, or where the system is not isolated. Understanding these scenarios is essential for accurately describing the rotational dynamics of physical systems and for applying the principles of angular momentum conservation in appropriate contexts.

Here's a detail that's worth remembering.