$A$ person driving a car suddenly applies the brakes on seeing a child on the road ahead. If he is not wearing a seat belt,he falls forward and hits his head against the steering wheel. Why?

(N/A) When a person driving a car suddenly applies the brakes,the lower part of the body,which is in contact with the seat,slows down along with the car due to the frictional force.
However,the upper part of the body continues to move forward due to the $inertia$ $of$ $motion$.
Since the person is not wearing a seat belt,there is no external force to stop the forward motion of the upper body,causing them to fall forward and hit their head against the steering wheel.