Question

why we fall backward when a bus accelerates to start

Answer 1

If a body is moving, then it tries to remain in motion. ... This is because when the bus accelerates, the inertia of the passenger tends to oppose the forward motion of the bus. Hence, the passenger tends to fall backwardswhen the bus acceleratesforward.

Answer 2

Because of the inertia. The lower part of the body starts moving with the bus whereas the upper part of our body is still at rest. That's the reason why we fall backwards when a bus accelerates.