Categories: Guide to Space

Law of Inertia

[/caption]In the world of physics, there are few people who have been more influential than Sir Isaac Newton. In addition to his contributions to astronomy, mathematics, and empirical philosophy, he is also the man who pioneered classical physics with his laws of motion. Of these, the first, otherwise known as the Law of Inertia, is the most famous and arguably the most important. In the language of science, this law states that: Every body remains in a state of constant velocity unless acted upon by an external unbalanced force. This means that in the absence of a non-zero net force, the center of mass of a body either remains at rest, or moves at a constant velocity. Put simply, it states that a body will remain at rest or in motion unless acted upon by an external and unbalanced force.

Prior to Aristotle’s theories on inertia, the most generally accepted theory of motion was based on Aristotelian philosophy. This ancient theory stated that, in the absence of an external motivating power, all objects on Earth would come to rest and that moving objects only continue to move so long as long there is a power inducing them to do so. In a void, no motion would be possible since Aristotle’s theory claimed that the motion of objects was dependent on the surrounding medium, that it was responsible for moving the object forward in some way. By the Renaissance, however, this theory was coming to be rejected as scientists began to postulate that both air resistance and the weight of an object would play a role in arresting the motion of that object.

Further advances in astronomy were another nail in this coffin. The Aristotelian division of motion into “mundane” and “celestial” became increasingly problematic in the face of Copernicus’ model in the 16th century, who argued that the earth (and everything on it) was in fact never “at rest”, but was actually in constant motion around the sun.Galileo, in his further development of the Copernican model, recognized these problems and would later go on to conclude that based on this initial premise of inertia, it is impossible to tell the difference between a moving object and a stationary one without some outside point of comparison.

Thus, though Newton was not the first to express the concept of inertia, he would later refine and codify them as the first law of motion in his seminal work PhilosophiaeNaturalis Principia Mathematica (Mathematical Principals of Natural Philosophy) in 1687, in which he stated that: unless acted upon by a net unbalanced force, an object will maintain a constant velocity. Interestingly enough, the term “interia” was not used in the study. It was in fact JohanneKepler who first used it in his Epitome AstronomiaeCopernicanae (Epitome of Copernican Astronomy) published from 1618–1621. Nevertheless, the term would later come to be used and Newton recognized as being the man most directly responsible for its articulation as a theory.

We have written many articles about the law of inertia for Universe Today. Here’s an article about Newton’s Laws of Motion, and here’s an article about Newton’s first law.

If you’d like more info on the law of inertia, check out these articles from How Stuff Works and NASA.

We’ve also recorded an entire episode of Astronomy Cast all about Gravity. Listen here, Episode 102: Gravity.


Matt Williams

Matt Williams is the Curator of Universe Today's Guide to Space. He is also a freelance writer, a science fiction author and a Taekwon-Do instructor. He lives with his family on Vancouver Island in beautiful British Columbia.

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