Explanation:
1.
Well the reason you feel the seat belt pressing on your chest, is because of something called inertia, and it's basically the resistance to changes in it's movement.
It's hard to start moving something, just like how it's hard to stop something that's moving, that's basically inertia.
In this case when the car comes to a sudden stop, your body has inertia, so it doesn't come to an immediate stop with the car simultaneously, but rather the car stops, and you keep moving forward, so you're going to feel the seat belt pressing on your chest.
2.
The main difference between being stopped by a seatbelt and being stopped by something like the steering wheel or dash of the motor, is that the force is more spread out, as well as the seatbelt not being completely rigid. This just means the seatbelt can stretch out more unlike the steering wheel or dash of the motor vehicle, so this gives you more time to come to a stop rather than something that's rigid and makes you stop faster, so essentially the force is spread out along more time.
Think of it like being stopped by a brick wall vs a net which stretches and allows more time for you to come to a stop, unlike a brick wall which almost instantly stops you.
3.
Well since the main thing that makes car crashes so dangerous is the sudden change in velocity, one thing that would make automobile crashes safer is slower speed limits... this may or may not decrease car crashes, but it will definitely make them a lot more safer. If you were to crash into another car at 5 mph, you're likely not going to get any injuries, unlike a crashing into a car at 90 mph.
