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
the rate of change of the angular momentum L equals the net torque N
and using the property of the vector cross product that v × w = 0 for any vectors v and w pointing in the same direction,
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.
Read more about this topic: Two-body Problem
Famous quotes containing the word motion:
“Theres not the smallest orb which thou beholdst
But in his motion like an angel sings,
Still quiring to the young-eyed cherubins;
Such harmony is in immortal souls,
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—William Shakespeare (15641616)