The Truck and Ladder
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.
For more information on physical descriptions of motion, visit The Physics Classroom Tutorial. Detailed information is available there on the following topics:
Newton's First Law of Motion
State of Motion
Balanced vs. Unbalanced Forces