Answer:
Newtons 1st law
Step-by-step explanation:
Newtons first law can be summarized by intertia; an object at rest will remain at rest, and an object in motion will remain in motion unless an unbalanced force is applied.
Lets say a car is moving at 25m/s. The person in the car is also moving at 25m/s. If the driver suddenly slams on the brakes, the car will stop moving, but the person will not stop, as they were already moving from before. There are no 'brakes' on a human, so the driver will smash through the windshield most likely.
The driver was already in motion, and when the car is at rest, for that little time, the human is not at rest, the human is still in motion so they will remain in motion, and the unbalanced force acted on them will be the windshield, or whatever it may be to cause the human to remain at rest.