Physics with Calculus/Mechanics/Relative Motion

In an approximation termed Galilean relativity, when two objects are moving at $$\vec{u}$$ and $$\vec{v}$$ relative to a common inertial reference frame, the relative velocity between the two objects, in the frame of reference where one of them is still, is $$\vec{v}_{\mathrm{ref}} = \vec{v} - \vec{u}$$