Relative velocity

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