Angular momentum is a measure of the rotational motion of an object or system, and like linear momentum, it is conserved in the absence of external torques. This principle states that in any isolated system—one free from outside forces or torques—the total angular momentum remains constant over time.
What Is Angular Momentum?
- Angular momentum (L) depends on:
- The mass of the object
- The distance from the axis of rotation
- The velocity of rotation or angular velocity
- For a spinning object or orbiting body, angular momentum reflects how difficult it is to change its rotational state.
Why It’s Conserved:
- Angular momentum conservation arises from rotational symmetry: if the laws of physics don’t change when you rotate the system, angular momentum must remain constant (as per Noether’s theorem).
- When no external torque acts on a system, any change in mass distribution must be balanced by a change in rotational speed to keep the total angular momentum the same.
Everyday Examples:
- Figure skaters spin faster when they pull in their arms — reducing their rotational radius increases spin speed to conserve angular momentum.
- Planets maintain stable orbits because their angular momentum in orbit is conserved unless disturbed.
- In astrophysics, collapsing stars form neutron stars or black holes that spin rapidly due to conservation of angular momentum.
Scientific Importance:
- Used to understand and predict the motion of celestial bodies, rotating machinery, atomic systems, and even subatomic particles.
- Fundamental in quantum mechanics, where particles like electrons have intrinsic angular momentum called spin.