# Understanding Inertia

Theories about motion started with Aristotle and he said that a projectile (an object thrown by a human) would move in a straight line, and when that “unnatural force” ran out, then natural motion would take over.  In other words any object that was thrown would continue along the path that it was on till it ran out of juice and then gravity would take over, however gravity was not defined till Newton came along.  The modern term inertia can be traced to its Latin roots which mean unskilled or artless.  Kepler first applied the word using it only for bodies at rest.  Galileo conducted experiments with falling objects or objects in motion and they became the cornerstone of physics.  Galileo discovered a very remarkable fact about motion, that if something is moving, with nothing touching it and completely undisturbed, it will go on forever, coasting at a uniform speed in a straight line. Galileo discovered the law of inertia, but did not name it.  Newton gave inertia its modern sense in the book he wrote ‘Principia’, where he said, “A body, from the inert state of matter, is not without difficulty put out of its state of rest or motion.”

In 1686, Newton presented his three laws of motion.  Newton’s first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force.  This is normally taken as the definition of inertia.  The key point here is that if there is no net force acting on an object (if all the external forces cancel each other out) then the object will maintain a constant velocity.  If that velocity is zero, then the object remains at rest.  The property of inertia concerns an object or a physical body that opposes any agency that attempts to put it in motion, or if it is already moving, inertia will restrict any change to the magnitude or direction of its velocity.  Inertia is a passive property and it does not enable a body to do anything except oppose such active agents as forces and torques.  A moving body keeps moving not because of its inertia, but only because of the absence of a force to slow it down, change its course, or speed it up.  If an external force is applied, the velocity will change because of the force.

Written for June 1, 2018 Fandango’s One-Word Challenge FOWC where the prompt is inertia

## 8 thoughts on “Understanding Inertia”

1. Thanks and it was my pleasure, thanks for hosting. What does your wife think of your new duties?

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1. She thinks I already spend too much time on WordPress, so I haven’t told her about FOWC yet.

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1. A very explanatory piece. Have you written textbooks?

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1. I wrote 3 books but none of them are published. My first book is a coffee table book titled “Man’s Footprint in Water”, my second is an adventure titled “Ancient Book of Eli” and my third book is a text book about mat titled :So You Want To Learn Calculus”.

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