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 this state of motion. This is often called the law of inertia.
The law of inertia is most commonly experienced when riding on the roadways. In fact, the tendency of moving objects to continue in motion is a common cause of a variety of transportation injuries - of both small and large magnitudes. Consider for instance the unfortunate collision of a motorcycle with a wall (or any obstacle inints path). Upon contact with the wall, an unbalanced force acts upon the motorcycle to abruptly decelerate it to rest. The rider of the motorcycle would also be decelerated to rest if strapped to the motorcycle by seat belts or some form of safety harness (which is not necessarily a good idea - please read on!). Being strapped tightly to the motorcycle, the driver would always share the same state of motion as the motorcycle. As the motorcycle accelerates, the rider accelerates with it. As the motorcycle decelerates, the rider decelerates with it. And as the motorcycle maintains a constant speed, the driver maintains a constant speed as well.
But what would happen if the rider were not wearing the seat belt? What motion would the rider undergo if she/he failed to use a seat belt and the motorcycle were brought to a sudden and abrupt halt by a collision with a wall or any other obstacle in its path? Were this scenario to occur, the driver would no longer share the same state of motion as the motorcycle. If a seat belt were used on a motorcycle, then the forces necessary for accelerated and decelerated motion would exist. Without a seat belt, the rider is more likely to maintain its state of motion. The animation below depicts this scenario.
If the motorcycle were to abruptly stop, then the rider in motion would continue in motion. The rider would likely be propelled from the motorcycle and be hurled into the air. Once they leave the motorcycle, the rider becomes a projectile and continues in projectile-like motion.
But why then are motorcycles not equipped with safety harnesses? Is this a gross oversight made by motorcycle manufacturers? Absolutely not! While no transportation accident is safe, it is the goal of the manufacturers of all roadway vehicles to produce a vehicle which maximizes the safety of its riders. In the case of a motorcycle, it is believed that the rider's safety is maximized by not strapping the rider to the motorcycle. In a car accident, the safest place to be is in the car; yet in a motorcycle accident, the worst place to be is on the motorcycle. The reason? Cars are four-wheeled vehicles which have a stable platform capable of resisting sideways motion and resisting tipping over. As such, being strapped to the a car in an accident is an advantageous strategy for maximizing passenger safety. On the other hand, a motorcycle is a single-track vehicle (two wheels) which are prone to tipping over and sliding into and underneath the obstacles which they hit. Imagine being strapped to your motorcycle as you slide underneath a 2000-pound car. Being strapped to the motorcycle by a safety harness, you would share the fate of the motorcycle itself - being crushed by the 2000-pound car. Your chance of survival would be minimal. On the other hand, if you were to leave the motorcycle and be flung into the air, it is more likely that your hopeful impact with the ground would slowly alter your velocity as you skid to a stop over a lengthened period of time. Motorcycles are inherently dangerous vehicles; yet like all vehicles, manufacturers design them in a manner that maximizes rider safety. The omission of safety harnesses from motorcycles means that the motorcyclist does not share the same fate as the tipped over and skidding single-track vehicle.
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