Question
Define angular momentum. Derive an expression for it in vector form.
Solution
The angular momentum of a rotating body is the measure of the quantity of angular motion contained in the body. It is also called the moment of linear momentum of the body.
Due to rotational inertia possessed by the body about the axis of rotation, the rotating body cannot change its angular momentum itself.
The torque produced by applied force on the rotating body changes its angular momentum and rate of change of angular momentum is equal to torque on the body.
i.e., 
Consider, a body moving in XY plane.
The torque on the body revolving in XY plane is,
τz =xFy-yFx ...(1)
Let px and vx be the X-component,
py and vy be the Y component of linear momentum & velocity of body in XY plane respectively.
Now, 
