Simplify the following text for a Grade 10 native English-speaking person.
Original: Angular momentum is important in physics because it is a conserved quantity : a system 's angular momentum stays constant unless an external torque acts on it .
Simplified: In physics , angular momentum , moment of momentum , or rotational momentum is a conserved vector quantity that can be used to describe the overall state of a physical system . The angular momentum L of a particle with respect to some point of origin is