A person driving a car suddenly applies the brakes on seeing a child on the road ahead

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

When a person driving a car suddenly applies the brakes, the lower part of the body slower down with the car while upper part of the body continues to move forward due to inertia of motion. If driver is not wearing seat belt, then he falls forward and his head hit against the steering wheel.