Relative velocity

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