The first term m v × v is always zero, because the vector cross product is always zero for any two vectors pointing in the same or opposite directions. However, when F is a central force, the remaining term r × F is also zero because the vectors r and F point in the same or opposite directions. Therefore, the angular momentum vector L is constant. Then r ⋅ ⋅ L = r ⋅ ⋅ ( r × × p ) = p ⋅ ⋅ ( r × × r ) = 0