car stopping suddenly.

a force or a mass

The car stopping suddenly is the result of a force acting upon it. When the driver applies the brakes, a force is generated between the brake pads and the car's wheels, causing a frictional force that opposes the motion of the car. This force ultimately brings the car to a stop. The mass of the car also plays a role in determining the stopping distance, as a larger mass requires a greater force to stop than a smaller mass.