Newton
In 1665, the plague broke out in London, forcing Isaac Newton to leave Trinity College in Cambridge, England, where he was a student. Over the next two years, Newton built on the work of scientists such as Galileo. He published his results many years later in a book entitled Principia. In this important work, Newton first had to define mass and force. He then introduced his laws of motion. Newton's portrait and the title page of Principia are shown in Figure 11.
Figure 11 Isaac Newton published his work on force and motion in the book entitled Principia.
Newton's First Law of Motion
Newton summarized his study of force and motion in several laws of motion. 
According to Newton's first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero. Thus, unless an unbalanced force acts, an object at rest remains at rest, and an object in motion remains in motion with the same speed and direction. For example, a soccer ball resting on the grass remains motionless until a force is applied to it in the form of a kick. The kicked ball begins rolling. Because friction between the grass and the ball acts on the ball as it rolls, the ball slows. The force of friction slows the ball and brings it to a stop.
Newton's first law of motion is sometimes called the law of inertia (in ur shuh). Inertia is the tendency of an object to resist a change in its motion. In other words, an object at rest tends to remain at rest, and an object in motion tends to remain in motion with the same direction and speed. Note that as the soccer ball sat motionless in the grass, the forces acting on it were balanced. The ball remained at rest until an unbalanced force acted on it. The ball has inertia.
How does a zero net force affect an object's motion?
Figure 12
A This crash sequence illustrates inertia—the tendency of an object in motion to remain in motion.
B At impact, the air bag deploys. Note that the test dummy continues its forward motion as the collision begins to slow the car.
C As the car comes to a stop, the air bag prevents the test dummy from striking the steering wheel.
D Interpreting Photographs What objects and devices absorb the energy of the crash?
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