Relative velocity

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${\displaystyle {\textbf {F}}={\frac {d}{dt}}(m{\textbf {v}})}$ Second law of motion
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The relative velocity, denoted ${\displaystyle {\vec {v}}_{B\mid A}}$ (also ${\displaystyle {\vec {v}}_{BA}}$ or ${\displaystyle {\vec {v}}_{B\operatorname {rel} A}}$), is the velocity vector of an object or observer B in the rest frame of another object or observer A. The relative speed ${\displaystyle v_{B\mid A}=\|{\vec {v}}_{B\mid A}\|}$ is the vector norm of the relative velocity.