Question

a passenger in a bus tends to fall backward when it starts suddenly. give reason

Answer 1

because when he sitted on bus his whole body is in rest but when bus started his bottom part of body start moving with the bus too the head would stay at rest causing it to fall backwards.

Answer 2

this is called inertia of rest
our feet which are in contact with bus start to move with bus
but upper part of body tends to remain at rest
that's why we fall backward