Question

when bus start person lean backward why ,

Answer 1

because bus start by much speed but passengers has in a satution rest

Answer 2

This is due to newton's 1st law of motion or inertia of rest of upper part of the passengers body when the bus suddenly starts moving, The lower part of the passenger’s body starts moving along with the bus or train while the upper part of the body has a tendency to remain in the state of rest due to inertia of rest, and hence falls backwards.
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