A plumb line is suspended from a ceiling of a car moving with horizontal acceleration of a. What will be the angle of inclination with vertical 

When car moves towards right with acceleration  a then due to pseudo force the plumb line will tilt in backward direction making an angle   with vertical.
 From the figure,$\mathrm{tan\theta }=\mathrm{a}/\mathrm{g}$
                          $\therefore \mathrm{\theta }={\tan }^{-1}(\mathrm{a}/\mathrm{g})$