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 are doing. All objects resist changes in
their state of motion. In the absence of an unbalanced force, an
object in motion will maintain its 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 a ladder
strapped to the top of a painting truck. As the truck moves down the
road, the ladder moves with it. Being strapped tightly to the truck,
the ladder shares the same state of motion as the truck. As the truck
accelerates, the ladder accelerates with it; as the truck
decelerates, the ladder decelerates with it; and as the truck
maintains a constant speed, the ladder maintains a constant speed as
well.
But what would happen if the ladder was negligently strapped to
the truck in such a way that it was free to slide along the top of
the truck? Or what would happen if the straps deteriorated over time
and ultimately broke, thus allowing the ladder to slide along the top
of the truck? Supposing either one of these scenarios were to occur,
the ladder may no longer share the same state of motion as the truck.
With the strap present, the forces exerted upon the car are also
exerted upon the ladder. The ladder undergoes the same accelerated
and decelerated motion that the truck experiences. Yet, once the
strap is no longer present, the ladder is more likely to maintain its
state of motion. The animation below depicts a possible scenario.
If the truck were to abruptly stop and the straps were no longer
functioning, then the ladder in motion would continue in motion.
Assuming a negligible amount of friction between the truck and the
ladder, the ladder would slide off the top of the truck and be hurled
into the air. Once it leaves the roof of the truck, it becomes a
projectile and continues in projectile-like motion.
