Question

If a stationary bus suddenly speeds up passengers are thrown in the backward direction

Answer 1

Answer:

inertia

Explanation:

when the bus suddenly starts to speed up, inertia acts on them thus they move backwards  

Answer 2

From Newtons First Law of Motion which states as below:

Everybody continues in its state of rest or of uniform motion in a straight line unless and until it is compelled by an external force to change that state.

When the bus is not moving the passengers are in the state of rest and they have inertia of rest. When the bus starts moving suddenly, the lower part of the body of passengers, which is in contact with the bus, come in motion, but upper part of their body tends to be in the state of rest and the passengers fall backward.