# Two-body Problem - Two-body Motion Is Planar

Two-body Motion Is Planar

The motion of two bodies with respect to each other always lies in a plane (in the center of mass frame). Defining the linear momentum p and the angular momentum L by the equations

$mathbf{L} = mathbf{r} times mathbf{p} = mathbf{r} times mu frac{dmathbf{r}}{dt}$

the rate of change of the angular momentum L equals the net torque N

$mathbf{N} = frac{dmathbf{L}}{dt} = dot{mathbf{r}} times mudot{mathbf{r}} + mathbf{r} times muddot{mathbf{r}} ,$

and using the property of the vector cross product that v × w = 0 for any vectors v and w pointing in the same direction,

$mathbf{N} = frac{dmathbf{L}}{dt} = mathbf{r} times mathbf{F} ,$

with F = μ d 2r / dt 2.

Introducing the assumption (true of most physical forces, as they obey Newton's strong third law of motion) that the force between two particles acts along the line between their positions, it follows that r × F = 0 and the angular momentum vector L is constant (conserved). Therefore, the displacement vector r and its velocity v are always in the plane perpendicular to the constant vector L.