What a seat belt does
Wearing a seat belt when travelling in a motor car is necessary in order to avoid being flung forwards when the brakes are applied.
What happened to the passenger on the bus happens to someone in a car too. Even though the car stops suddenly, by the first law the body tries to carry on forwards at the velocity it had.
What the seat belt does
The seat belt applies a force to the upper part of the passenger's body as well.
So when the brakes are applied, the whole body stays at the velocity of the vehicle — not just the lower half. The body is not split in two.
The part that is easy to miss
The seat belt does not stop the passenger. What it does is stop them at the same rate the car stops. Without one, the body stops only when it meets the windscreen or the steering wheel — that is, in a very short time indeed.
Recall chapter 2
The same change in velocity, spread over a shorter time, means a bigger retardation. And a big retardation means a big force — the next section shows how to work that out.