Why does it happen? If a stationary bus suddenly speeds up, passengers are thrown in the backward direction.

Hint: According to Newton's first law a body at rest will remain at rest and a body in motion will remain in motion unless or until an external force is applied to it. This law is also called the law of inertia, where inertia is the tendency of a body to resist any change in its rest or motion. With the help of this, we can explain the given situation.

Complete answer:
When a passenger is sitting in a stationary bus it is under the law of inertia of rest that is the body resists any change in its state of rest. When the bus from stationary suddenly speeds up it changes its inertia of rest, since our feet are touched with the bus’s floor so the lower half of the body comes in motion but not the upper half of the body. Due to this we feel a sudden jerk and are thrown in the backward direction because our inertia of rest changes to inertia of motion.

Hence, we can say due to the law of inertia if a stationary bus suddenly speeds up, passengers are thrown in the backward direction.

Note: There are three types of inertia:
The inertia of rest- due to which a resting body resists any change in its state of rest.
The inertia of motion- due to which a moving body resist any change in its state of motion
The inertia of direction- due to which a body moves in a particular direction unless an external force is applied to it.