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Inertia is the name for the tendency of an object in motion to remain in motion, or an object at rest to remain at rest unless acted upon by a force. This concept was quantified in Newton's First Law of Motion.
The word inertia came from the Latin word iners, which means idle or lazy and was first used by Johannes Kepler.
Inertia and Mass
Inertia is a quality of all objects made of matter that possess mass. They keep doing what they are doing until a force changes their speed or direction. A ball sitting still on a table won't start rolling around unless something pushes on it, be it your hand, a gust of air, or vibrations from the surface of the table. If you tossed a ball in the frictionless vacuum of space, it would travel on at the same speed and direction forever unless acted on by gravity or another force such as a collision.Volker Möhrke / Getty Images
Mass is a measure of inertia. Objects of higher mass resist changes in motion more than objects of lower mass. A more massive ball, such as one made of lead, will take more of a push to start it rolling. A styrofoam ball of the same size but low mass may be set in motion by a puff of air.
Theories of Motion From Aristotle to Galileo
In everyday life, we see rolling balls come to rest. But they do so because they are acted on by force of gravity and from the effects of friction and air resistance. Because that is what we observe, for many centuries Western thought followed the theory of Aristotle, who said that moving objects would eventually come to rest and needed continued force to keep them in motion.
In the seventeenth century, Galileo experimented with rolling balls on inclined planes. He discovered that as friction was reduced, balls rolled down an inclined plane attained almost the same height rolling back up an opposing plane. He reasoned that if there were no friction, they would roll down an incline and then keep rolling on a horizontal surface forever. It wasn't something innate in the ball that caused it to stop rolling; it was contact with the surface.
Newton's First Law of Motion and Inertia
Isaac Newton developed the principles shown in Galileo's observations into his first law of motion. It takes a force to stop the ball from continuing to roll once it is set in motion. It takes a force to change its speed and direction. It doesn't need a force to continue moving at the same speed in the same direction. The first law of motion is often referred to as the law of inertia. This law applies to an inertial reference frame. Corollary 5 of Newton's Principia says:
The motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forwards in a straight line without circular motion.
In this way, if you drop a ball on a moving train that is not accelerating, you will see the ball fall straight downward, as you would on a train that was not moving.