Inertia is the reason that people in cars need to wear seat belts. A
moving car has inertia, and so do the riders inside it. When the driver
applies the brakes, an unbalanced force is applied to the car. Normally,
the bottom of the seat applies an unbalanced force—friction—which
slows the riders down as the car slows. If the driver stops the car suddenly, however, this force is not exerted over enough time to stop the
motion of the riders. Instead, the riders continue moving forward with
most of their original speed because of their inertia.
