A simple pendulum with a bob of mass m is suspended from the roof of a car moving with a horizontal acceleration a.
 

The string makes an angle of ${\tan }^{-1}\left(\frac{\mathrm{a}}{\mathrm{g}}\right)$ with the vertical.

The tension in the string is $\mathrm{m}\sqrt{{\mathrm{g}}^2-{\mathrm{a}}^2}$

The string makes an angle of ${\tan }^{-1}\left(\frac{\mathrm{g}}{\mathrm{a}}\right)$  with the vertical. 

The tension in the string is  $\mathrm{m}\sqrt{{\mathrm{g}}^2+{\mathrm{a}}^2}$