Chapter 21, Problem 66P

(III) Suppose a dipole $\stackrel{\to }{p}$ is placed in a nonuniform electric field $\stackrel{\to }{E}=E\hat{i}$ that points along the x axis. If $\stackrel{\to }{E}$ depends only on x, show that the net force on the dipole is

$\stackrel{\to }{F}=\left(\stackrel{\to }{p}\cdot \frac{d\stackrel{\to }{E}}{dx}\right)\hat{i},$