The forces that make a passenger speed up, slow down, or
turn a curve are the same forces that have the same effect
on the driver and anybody else in the car.
-- Speeding up . . .
the back of the seat
friction between the car seat and the seat of your pants
-- Slowing down . . .
the seat belt
friction between the car seat and the seat of your pants
-- Turning away from a straight line . . .
the seat belt
friction between the car seat and the seat of your pants
the door, or whatever or whomever you're leaning against