What provides the force on the person on the passenger seat?

What do u mean? Do you mean what allows you to let you sit on the passengers seat?

When the car speeds up, slows down, or goes around a curve,

passengers need a force applied to them to make them do the

same thing, otherwise they won't keep up with the car.

The force on the passenger is applied by means of friction between

the upholstery and the seat of his pants, and also by the seat-back

or his seat-belt.