Answer:
The answer is law of inertia
Step-by-step explanation:
The law of inertia is the first law in Newton's laws of motion. It postulates that an object or body at a state of rest or in uniform motion continue in such state unless an external force is applied.
Brennan is a body, in a car that is moving at a constatnt speed. The moment an external force (brakes) is applied to the car's speed, Brennan's upper body which was at a state of rest starts moving forward.
Likewise, it takes the application of another force (acceleration) to restore Brennan's upper body back to the state it was before the car's breakes were applied.