Seat belts and air bags save lives by reducing the forces exerted on the driver and passengers in an automobile colli- sion. Cars are designed with a "crumple zone" in the front of the car. In the event of an impact, the passenger compartment decelerates over a distance of about 1 m as the front of the car crumples. An occupant restrained by seat belts and air bags decelerates with the car. By contrast, an unrestrained occu- pant keeps moving forward with no loss of speed (Newton's first law!) until hitting the dashboard or windshield, as we saw in Figure 4.2. These are unyielding surfaces, and the unfortunate occupant then decelerates over a distance of only about 5 mm.

A 60 kg person is in a head-on collision. The car's speed at impact is 15 m/s.

Estimate the net force on the person if he or she is wearing a seat belt and if the air bag deploys.

Estimate the net force that ultimately stops the person if he or she is not restrained by a seat belt or air bag.

How do these two forces compare to the person's weight?