The driver and passengers advance together. When the driver suddenly braked, the passengers had to keep the original motion state and move forward because of inertia, so it is wrong to say that they suddenly lost inertia.

Because inertia is an attribute of the object itself, the object will not change its inertia because of the change of motion or speed, and inertia is only related to the quality of the object itself.

So the answer is: the passenger was injured because he suddenly lost his inertia and rushed forward; Because inertia is an attribute of the object itself, the object will not change its inertia because of the change of motion or speed, and inertia is only related to the quality of the object itself.