In one concise sentence

MATHEMATICS · Subjective · AP · Calculus BC · Parametric, Polar, and Vector-Valued Functions · Parametric, Polar, and Vector-Valued Functions

Problem

In one concise sentence, explain why ∣r(t)∣|\mathbf r(t)| remains constant for all tt if ddt[r(t)⋅r(t)]=0\dfrac{d}{dt}[\mathbf r(t)\cdot\mathbf r(t)]=0 on an interval.

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