Final answer:
When a car stops suddenly, Newton's 3rd Law of Motion comes into play. Your body continues to move forward, exerting a force on the car, and the car exerts an equal and opposite force on your body.
Step-by-step explanation:
Newton's 3rd Law of Motion states that for every action, there is an equal and opposite reaction. When a car suddenly stops, your body continues to move forward due to inertia. As your body collides with the dashboard or windscreen, it exerts a force on the car, and according to Newton's 3rd Law, the car exerts an equal and opposite force on your body.
Learn more about Newton's 3rd Law of Motion