# Physics with Calculus/Mechanics/Relative Motion

In an approximation termed Galilean relativity, when two objects are moving at ${\displaystyle {\vec {u}}}$ and ${\displaystyle {\vec {v}}}$ 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
${\displaystyle {\vec {v}}_{\mathrm {ref} }={\vec {v}}-{\vec {u}}}$