Relative speed

The relative velocity ${\displaystyle {\vec {v}}_{\mathrm {B|A} }}$ (also ${\displaystyle {\vec {v}}_{\mathrm {BA} }}$ or ${\displaystyle {\vec {v}}_{\mathrm {BrelA} }}$) is the velocity of an object or observer B in the rest frame of another object or observer A.

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