Velocity and Speed

Core Sentence / 核心句

Velocity is the rate of change of position; instantaneous speed is the magnitude of instantaneous velocity.

中文理解：velocity 带方向，speed 只保留大小。

v_{avg} = \frac{Δ r}{Δ t}, v = \frac{d r}{d t}, speed = ∣ v ∣

average speed = \frac{total distance}{Δ t}

Instantaneous speed $= ∣ v ∣$ ，但 average speed 通常不等于 $∣ v_{avg} ∣$ 。