Inertia, << ihn UR shuh, >> is a property of all matter. Due to inertia, an object’s state of motion does not change unless a force acts upon the object. An object that is not moving remains motionless unless a force puts the object into motion. In addition, a moving object continues to move at a constant speed and in the same direction unless a force changes the object’s motion. Only a force can make a moving object slow down, speed up, stop, or turn. One type of force that commonly slows or stops a moving object is friction with other objects.
The force required to change an object’s motion depends on the mass of the object. Mass may be defined as the amount of matter in an object. The greater an object’s mass, the harder it is to put the object into motion or to change its direction or speed. For example, a locomotive has more mass than an automobile. Therefore, it takes more force to stop a locomotive than to stop an automobile in the same amount of time when both are traveling at the same speed. Because of the relationship between inertia and mass, physicists usually define mass as a measure of inertia rather than as a measure of matter.
The difficulty involved in changing the direction or speed of an object also depends on how quickly the change is made. It is harder to slow down, speed up, or turn a moving object suddenly than to make the same change gradually. An automobile has more difficulty holding the road on a curve at high speed than at low speed. Physicists use the term acceleration to describe the rate of change in an object’s direction or speed.
The English scientist Sir Isaac Newton first described inertia. He introduced the idea in his first law of motion, which was published in 1687.
