A person in a car tends to fall back when it suddenly starts. It is due to

A person in a car tends to fall back when it suddenly starts. It is due to

inertia of rest.

i.e. when bus is suddenly accelerated then the person resist to change the state of rest and continue in the same state of rest, this will appears as if the person is falling back. We know that nobody can change its state of motion on its own.