The period of oscillation of a simple pendulum of length l suspended from the roof of the vehicle which moves down without friction on an inclined plane of inclination α , is given by:

Since, the vehicle is moving down the plane with an acceleration g sinα  and the bob  will experience a pseudo force up along the plane mg sinα                                                        effective acceleration of the pendulum is g cosαThe period of oscillation of the pendulum,                                                         T=2πlg'=2πlgcosα