\[\mathbf{F}\]
is a conservative force field then there exists a function \[\phi(x,y,z)\]
such that \[\mathbf{F}= - \mathbf{\nabla} \phi\]
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Take two points \[A,B\]
Newton's Second Law gives
\[\mathbf{F} = m \frac{d^2 \mathbf{r}}{dt^2} \rightarrow \mathbf{F} \cdot \frac{d \mathbf{r}}{dt} =m \frac{d^2 \mathbf{r}}{dt^2} \cdot \frac{d \mathbf{r}}{dt}\]
We can rewrite this as
\[ \mathbf{F} \cdot \frac{d \mathbf{r}}{dt} =m\frac{1}{2} \frac{d}{dt} ((\frac{d \mathbf{r}}{dt})^2)= \frac{1}{2} \frac{d (v^2)}{dt}\]
Now integrate
\[\int^B_A \mathbf{F} \cdot d \mathbf{r} = \int^B_A d(\frac{1}{2} mv^2 ) \rightarrow \phi_B - \phi_A =\frac{1}{2} m (V_B^2 - v_A^2 )\]
.This equation can be rearranged to give
\[\phi_A + \frac{1}{2}mv_A^2 = \phi_B + \frac{1}{2}mv_B^2\]