The period of oscillation of a simple pendulum of length L suspended from the roof of a vehicle which moves without friction down an inclined plane of inclination , is given by
See the following force diagram.
Vehicle is moving down the frictionless inclined surface so, it's acceleration is . Since vehicle is accelerating, a pseudo force will act on bob of pendulum which cancel the component of weight of the bob.
Hence net force on the bob is Fnet or net acceleration of the bob is
Time period