Angular momentum, a fundamental concept in physics, describes the rotational inertia of a spinning or orbiting object. It plays a significant role in various phenomena, from planetary motion to everyday activities. This physical quantity is conserved, but only under specific conditions. Understanding these conditions explains why some rotating systems maintain their spin while others change.
Understanding Angular Momentum
Angular momentum measures an object’s tendency to continue rotating. It is the rotational counterpart to linear momentum. It depends on how quickly an object is spinning (angular velocity) and how its mass is distributed around its axis of rotation (moment of inertia). Objects with mass farther from their axis have a greater moment of inertia, making them harder to turn.
The Conservation Rule
Conservation in physics means a quantity remains constant over time within a system. Angular momentum remains constant unless an external net torque acts upon it. Torque is a twisting force that causes or changes rotational motion. If the sum of all external torques on a system is zero, its total angular momentum does not change. This principle applies to isolated systems.
Angular Momentum in Action
Many everyday observations and natural phenomena demonstrate the conservation of angular momentum. An ice skater performing a spin provides an example: when she pulls her arms and legs closer to her body, her moment of inertia decreases, causing her to spin faster while her angular momentum remains constant. A diver tucks their body during a somersault to increase their spin rate, then extends to slow down before entering the water, conserving angular momentum. Planets orbiting the Sun also exhibit this conservation; as a planet moves closer to the Sun, its orbital speed increases, and it slows down as it moves farther away, maintaining constant angular momentum. Gyroscopes maintain their orientation due to the conservation of their angular momentum, resisting changes to their axis of rotation.
When Conservation Isn’t Observed
Angular momentum is not conserved when an external net torque acts on a system. This external torque causes a change in the system’s angular momentum. For instance, applying the brakes to a spinning bicycle wheel introduces an external torque due to friction, which reduces the wheel’s angular momentum until it stops rotating. A spinning top eventually slows down and falls due to air resistance and friction, both of which exert external torques. When a person jumps off a merry-go-round, they exert a torque, causing its angular momentum to change.