The Car and The Wall
A GIF Animation
According to Newton's first law, an object in motion continues in motion with
the same speed and in the same direction unless acted upon by an unbalanced force. It is
the natural tendency of objects to keep on doing what they're doing. All objects resist
changes in their state of motion. In the absence of an unbalanced force, an oject in
motion will maintain this state of motion. This is often called the law of inertia.
The law of inertia is most commonly experienced when riding in cars and
trucks. In fact, the tendency of moving objects to continue in motion is a common cause of
a variety of transportation accidents - of both small and large magnitudes. Consider for
instance the unfortunate collision of a car with a wall. Upon contact with the wall, an
unbalanced force acts upon the car to abruptly decelerate it to rest. Any passengers in
the car will also be deccelerated to rest if they are strapped to the car by seat belts.
Being strapped tightly to the car, the passengers share the same state of motion as the
car. As the car accelerates, the passengers accelerate with it; as the car decelerates,
the passengers decelerate with it; and as the car maintains a constant speed, the
passengers maintain a constant speed as well.
But what would happen if the passengers were not wearing the seat belt? What
motion would the passengers undergo if they failed to use their seat belts and the car
were brought to a sudden and abrupt halt by a collision with a wall? Were this scenario to
occur, the passengers would no longer share the same state of motion as the car. The
presence of the strap assures that the forces necessary for accelerated and decelerated
motion exist. Yet, once the strap is no longer present to do its job, the passengers are
more likely to maintain its state of motion. The animation below depicts this scenario.
If the car were to abruptly stop and the seat belts were not being worn, then
the passengers in motion would continue in motion. Assuming a negligible amount of
friction between the passengers and the seats, the passengers would likely be propelled
from the car and be hurled into the air. Once they leave the car, the passengers becomes
projectiles and continue in projectile-like motion.
Now perhaps you will be convince of the need to wear your seat belt. Remember
it's the law - the law of inertia.