##### 挠率 - 挠率 - 挠率是描述空间曲线在某点的扭曲程度的几何量 - $\displaystyle\tau=-\frac{{\rm d}\mathbf{B}}{{\rm d}s}\cdot\mathbf{N}$ - $\displaystyle\tau = \frac{\left( {\mathbf{r}'(t) \times \mathbf{r}''(t)} \right)\cdot \mathbf{r}'''(t)} {\left\| {\mathbf{r}'(t) \times \mathbf{r}''(t)} \right\|^2}$