In a straight-line motion, how are speed and velocity related?

• A. Velocity is the magnitude of the speed vector, so $v=\left|\mathrm{speed}\right|$.
• B. Speed is a vector and velocity is a scalar; they differ only by units.
• C. Speed is the magnitude of the velocity vector; equivalently, $\mathrm{speed}=\left|v\right|$.
• D. Speed and velocity are always identical because both include direction.

Answer: (C)