As an object is moving or resting in space it will resist any change towards its motion such as its speed, direction, or state of rest. This is due to the objects inertia; which can be described as an objects resistance to change in motion. Inertia is the reason why moving objects will want to travel in a straight line, and why when an object is going around a corner it develops a normal force even if the velocity of that object is constant. The resistance caused by “inertia” is directly related toward that objects mass or momentum. This in turn is where the equation of motion comes from since a force is generated by mass times acceleration and the more massive an object is the more force it will be required to accelerate or decelerate that object.
History of Inertia
Before the Renaissance the accepted theory of motion came Aristotle who stated that “in the absence of an external motive power, all objects (on Earth) would come to rest and that moving objects only continued to move so long as there is power inducing them to do so.” This statement was generally accepted for a couple millennia. However, there were a few notable philosophers that disputed this statement. Lucretius stated that the default state of matter was actually motion instead of stasis. While another philosopher John Philoponus criticized Aristotles theory on projectiles, which stated that the medium that the projectile is traveling created the objects motion and that if that object entered a void the projectiles motion would stop. Philoponus instead stated that a medium did not control an objects motion but instead imparted to the object while it was in motion. However, power was still required to keep an object in motion. Next Johannes Kepler defined inertia only as a “term of resistance to a movement, that is based on a presumption that rest was a natural state that didn’t need an explanation.”
It wasn’t until Nicolaus Copernicus and Galileo that the theories of motion and the theories of inertia started to head towards where they are today. Copernicus correctly stated in the 16th century that the earth was not at rest and was actually in constant motion around the sun. Galileo took Copernicus’s theory even further. He observed that an object that is in motion on a level surface will remain at a constant speed unless that object is disturbed. Galileo also observed that a person cannot observe that an object is in motion unless they visually so that the object was in motion. The observation lead to the initial premise of inertia.
From Galileo’s finding the first physicist that completely disregarded Aristotle’s theory of motion was Isaac Beckman in 1614. He stated “unless acted upon by a net unbalanced force, an object will maintain a constant velocity.” This statement would later be used in Newton’s laws of motion where the velocity is defined as vector, which means that constant speed also applies a constant direction. While zero speed means not motion.
Finally, Newton came along to define his three laws of motion which is what is excepted today. However notice that Newton never actually used the term “inertia” in any of his three laws. Instead Newton originally observed the phenomenon of inertia as “an innate force inherent in matter which resisted acceleration.” Due to this observation and the information Newton had borrowed from Kepler, Newton described inertia “as the force possessed by an object which resists changes in motion.” This meant that inertia to Newton was a cause of the phenomenon instead of the phenomenon itself. This was problematic for a variety of reasons. Which is why today physicist do not think in these terms but instead describe inertia as an object ability to resist change in motion whether that be direction, speed, or state of rest, where inertia is ultimately described by Newton’s first law of motion.