Glossary · Vật lý
Angular momentum
A physical quantity that describes how an object rotates about a point or an axis; it is a vector. When no net external torque acts, a system's angular momentum is conserved, so a body that draws its mass closer to the axis spins faster.
For a point particle, angular momentum about a point is the cross product of its position vector and its linear momentum: L = r × p. For a rigid body rotating about an axis, it equals the moment of inertia multiplied by the angular velocity: L = Iω.
The law of conservation of angular momentum states that if the net external torque about a point is zero, the system's angular momentum about that point stays constant. So when the moment of inertia decreases, as when a figure skater pulls in their arms and legs, the angular velocity must increase.
In astronomy, the principle is used to explain why a rotating gas cloud spins faster as it contracts under gravity, which is part of the model for how the Solar System formed.