什么是 Scalar multiple？

Let $\vec u$ be a nonzero vector and $k$ a real number. The vector $k\vec u$ has the same direction as $\vec u$, length $\abs{k} \times \norm{\vec u}$, and points the same way as $\vec u$ if $k > 0$, the opposite way if $k < 0$. For $k = 0$, $0\vec u = \vec 0$.