We are given the following information
Mass = m = 45.0 kg
Initial speed = vi = 40.0 m/s
Time = t = 0.500 s
We are asked to find the average force that the seat belt exerted on her.
Recall from Newton's second law of motion,
Where a is the acceleration of the car and is given by
Where vf is the final velocity of the car that must be 0 since the car was stopped.
The negative sign indicates deacceleration since the card was stopped.
So, the force is
Therefore, the seat belt exerted a force of 3600 N on her.