As an object is moving through space it will resist any change of motion. This is due to the objects inertia. An objects inertia is the ability that object has to resist changes in motion. Inertia is the reason why moving objects will want to travel in a straight line. It is also 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. From this we can derive the equation of motion from a particle mass and acceleration.
(Eq 1) $F=ma$
History of Inertia
Prior to the Renaissance people accepted Aristotle’s theory of motion. It stated “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”. It was a couple of millennia before other philosophers began to challenge this statement. A few notable philosophers that disputed this statement are as follows.
Lucretius stated that the default state of matter was actually motion instead of stasis. While another philosopher John Philoponus criticized Aristotles theory on projectiles. This theory 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, to keep the object in motion power would still be required. 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 started to head towards where they are today. Copernicus correctly stated in the 16th century that the earth was not at rest. Instead it was actually in constant motion around the sun. Galileo took Copernicus’s theory even further. He states that an object in motion on a level surface will remain at a constant speed unless it 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.
After taking Galileo’s finding into consideration, 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”. Later Newton will incorporate Beckman’s statement into his laws of motion. This done by stating that velocity is a vector. Due to this there will be a velocity and the velocity will have a certain direction.
Finally, Newton stated his three laws of motion. However, in the process of doing this 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. As a result physicist today describe inertia as the ability the object has to resist change.