To demonstrate this mathematically, it suffices to show that the angular momentum of the particle is constant. This angular momentum L is defined by the equation L = r × × p = r × × m v where m is the mass of the particle and p is its linear momentum. In this equation the times symbol × indicates the vector cross product, not multiplication. Therefore, the angular momentum vector L is always perpendicular to the plane defined by the particle's position vector r and velocity vector v.