Newton's Laws
The Car and The
Wall
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 object 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.
For more information on physical descriptions of motion, visit
The Physics Classroom Tutorial.
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