-\frac{\text{d}V}{\text{d}\mathbf{r}}\cdot \frac{\text{d}\mathbf{r}}{\text{d}t}, \quad \Rightarrow \frac{\text{d}}{\text{d}t} \left[\frac{m}{2}\left( \frac{\text{d}\mathbf{r}}{\text{d}t} \right)^2 + V\right] = \frac{\text{d}}{\text{d}t}(T+V) = 0