Isaac Newton's laws of motion have become the backbone of classical physics. These laws, first published by Newton in 1687, still accurately describe the world as we know it today. His First Law of Motion states that an object in motion tends to stay in motion unless another force acts upon it. This law is sometimes confused with the principles in his second law of motion, which states the relationship among force, mass and acceleration. In these two laws, however, Newton discusses separate principles that, although often intertwined, nevertheless describe two different aspects of mechanics.
Balanced vs. Unbalanced Forces
Newton's first law deals with balanced forces, or those that are in a state of equilibrium. When two forces are balanced, they cancel one another and have no net effect on the object. For example, if you and your friend both pull on opposite ends of a rope using an equal amount of force, the center of the rope will not move. Your equal, but opposite forces cancel one another. Newton's second law, however, describes objects affected by unbalanced forces, or forces that do not cancel. When this occurs, there is net movement in the direction of the more powerful force.
Inertia vs. Acceleration
According to Newton's first law, when all the forces that are at work on an object are balanced, that object will remain in the state that it is in forever. If it is moving, it will remain moving at the same speed and in the same direction. If it is not moving, it will never move. This is known as the Law of Inertia. According to Newton's second law, if the status quo changes so that the forces at work on the object become unbalanced, the object will accelerate at a rate described by the equation F = ma, where "F" equals the net force acting upon the object, "m" equals its mass and "a" equals the resulting acceleration.
Unconditional vs. Conditional State
Inertia and acceleration describe different properties of the object. Inertia is an unconditional property that every object has at all times, regardless of what happens to it. An object, however, does not always accelerate. This happens only under a specific set of conditions; therefore, you can describe acceleration as a conditional state. The rate of acceleration is also conditional, in that it depends upon the object's mass and the amount of net force. For example, a 1-newton force acting upon a ball weighing 1 g will not cause the ball to accelerate as much as a 2-newton force.
Inertia describes why people in a moving vehicle must be restrained. If the car should stop suddenly, the people inside will continue moving forward unless a seat belt applies an opposing force. Acceleration describes why the car came to a sudden stop. Because deceleration is negative acceleration, it is governed by the second law. When the force opposing the car's forward motion became greater than the one propelling its motion, the car decelerated until it stopped.