Print in bold, and add an arrow to each letter to represent the vector, where "×" represents the vector product, which conforms to the right-handed spiral law.
Angular momentum refers to a certain center or axis of rotation of an object, and it is meaningless to talk about angular momentum without this center.
The direction of vector diameter is the position from the center to the object (position vector), and the vector diameter is the distance from the center to the object position; P is the momentum vector of the object at this position.
Direction of angular momentum: angular momentum is the cross product of two vectors. In the right-hand coordinate system, follow the right-hand spiral law, that is, the four fingers of the right hand point to the vector diameter direction, and after turning a plane angle less than 180 degrees, the four fingers point to the momentum direction, so the direction pointed by the thumb is the angular momentum direction.
Extended data:
Angular momentum is a vector, and its projection on the axis passing through point O is the angular momentum (scalar) of a particle on the axis. The angular momentum of a particle system or rigid body to a point (or axis) is equal to the vector (or algebraic) sum of the momentum of each particle to the point (or axis).
The geometric meaning of angular momentum is twice the area velocity times the vector diameter times the mass. The law of conservation of angular momentum points out that when the resultant torque is zero, the area swept by the connecting line between the object and the central point in unit time remains unchanged, which is manifested as Kepler's second law in celestial motion.
Angular momentum and angle are a pair of yoke physical quantities in quantum mechanics. Angular momentum is a concept corresponding to momentum in rigid body dynamics, and its magnitude depends on the rotating speed and the mass distribution of rotating objects.
In general, angular momentum and angular velocity are in the same direction, but in more general cases, it is not necessary to be in the same direction, even when the rigid body rotates around a fixed axis (this can be proved by the triple vector product algorithm of vectors).