Angular momentum is calculated with respect to a reference point. If the displacement vector from a reference point to a body is r and the body has momentum p , then the body's angular momentum with respect to that point is, using the vector cross product, L = r × × p . Taking the time derivative of the angular momentum gives d L d t = ( d r d t ) × × p + r × × d p d t = v × × m v + r × × F . The first term vanishes because v and m v point in the same direction. The remaining term is the torque, τ τ = r × × F . When the torque is zero, the angular momentum is constant, just as when the force is zero, the momentum is constant. The torque can vanish even when the force is non-zero, if the body is located at the reference point ( r = 0 ) or if the force F and the displacement vector r are directed along the same line.