Why does stopping a vehicle you’re in cause you to jerk forward, even though motion is supposed to be relative (and the vehicle isn’t moving from your “physics” perspective)?

Question

Why does stopping a vehicle you’re in cause you to jerk forward, even though motion is supposed to be relative (and the vehicle isn’t moving from your “physics” perspective)?

In: Physics

Answer 1

If you’re in a car going 20 mph, both the car and your body are also going 20 mph. If you decelerate quickly to 0 with the brake, that only stops the car. You continue to travel at 20 mph until your seat belt stops you. If no seatbelt, you burst through the window shield and land on the concrete.