Universal Physics Journal
Article PR1

Is Inertia Real or Imaginary?

     Is inertia actually anything real? Do objects possess an inertia that affects their behavior? I wonder. Come join with me on this journey in search of the truth.

     Inertia is generally professed to be a non-variable, forceless, ever-present, property of matter that serves to maintain an object's inactive state of motion (rest or uniform motion in a straight line) by resisting any attempt by an action force to change the object to its active state of motion (acceleration).

Does Inertia Maintain an Object's Inactive State of Motion (Rest-motion)?
     This role for inertia is assigned in PRINCIPIA in Definition IV by Isaac Newton [1]. Does this make any sense to you? In my understanding, supported by Newton's own LAW I, the only way an object can ever be caused to accelerate (change its motion) is when an acceleration-causing, action force is present. Conversely, when such an action force is absent, acceleration of the object is always absent. Thus the simple absence of such an action force leaves the object with no other choice but to wait in the inactive state of motion (rest or uniform motion, known as rest-motion) for the next acceleration-causing action force to come along. I see this inactive state as the object's default state of motion for which no cause exists. Hopefully you too can see that the only condition necessary to "maintain" the object's inactivity is the continued absence of an acceleration-causing action force. Here I see no role for Newton's inertia to fill.

Does Inertia Resist An Object's Active State of Motion (Acceleration)?
     When an acceleration-causing action force does become present, acceleration for the object is immediate, in the general direction of the action force, and at a rate predicted by Newton's formula, a = F / m (acceleration = Force/mass). The object contains matter. It takes the application of an action force to cause acceleration for this matter. Due to the mutual nature of forces as stated in Newton's LAW III, and in Rule 7, part b of Article 10,  it is not possible to apply an acceleration-causing action force to the object's matter without there immediately becoming present an equal and opposite acceleration-caused reactionary support force.  If the acceleration/Action force is an external (contact) force from a second object, the acceleration/Reaction support force from the accelerating object can be measured at the mutual point of contact. This variable acceleration/Reaction force is really just a support force since it is caused by, and remains equal in magnitude and opposite in direction to, the variable acceleration/Action force.  [2]

     Here the acceleration/Reaction force being reactively generated within the object's matter is measurable and variable and is reduced to zero whenever the acceleration/Action force and the object's resulting rate of acceleration are reduced to zero. None of these qualities are inertia qualities since inertia is professed to be an ever-present property (never zero) that is non-variable (never variable) and forceless (never forceful). Thus inertia is predicted to be quite different from the measurable, variable, acceleration/Reaction support force being reactively generated within the accelerating object's matter.  Again I see no role for Newton's inertia to fill. What do you see?

     What role is left for us to consider? Perhaps inertia does represent a "resistance" to an object's acceleration that needs to be overcome. First of all, I see "resistance" as being no help in proving inertia's existence since use of this term alters inertia's definition from being forceless to instead being forceful.  After all, how can a "resistance" to an object's acceleration exist without being a force of opposition like the force of friction?  Other than friction, I know of no counter force that exists as a "resistance" that slows or prevents the acceleration of an object.  As an example, try standing while holding a rock out at arm's length. Now release the rock. Note how the rock's acceleration from rest to motion being caused by the downward-directed, internal force of gravitation is immediate. Film this event with a high-speed movie camera.  You will see that there is no hesitation that might indicate the presence of some kind of "resistance" to acceleration. Upon release, the rock's acceleration is not only immediate but is also at its highest rate during its entire fall to Earth. With this event as evidence, I see no reason Newton's formula need contain a fudge factor indicating that a = F/m holds true only after a short delay while the "resistance" of the accelerating object's "inertia" is being "overcome". 

    Often one reads where a cup full of water is used to demonstrate the water's inertial "tendency" to "resist" changes in motion when the cup is placed inside an accelerating vehicle.  Surely here inertia's presence will become convincingly clear.  First the car is stationary on level ground.  The water inside the cup is also level.  Now the driver accelerates to car to a faster and faster speed along level ground.  The water inside the accelerating cup is no longer level.  Instead, it slopes up or piles up at the rear of the cup with some of it sloshing over the rim.  Here such demonstrations are used to support the claim that the water's inertial "tendency" to keep doing what it is doing is causing the water to escape from the cup.  Is this true?  Is this water stubbornly refusing to participate in the accelerational event due to its inertial "tendency" as is claimed?  Or is there a simpler explanation where the invention of an "inertial tendency" is unnecessary? 

     If a cap is placed on the cup, during acceleration as before, the water will pile up against the cup's wall opposite to the acceleration but will be prevented by the cap from escaping over the rim.  Once the initial piling is done, the water ends up accelerating at the same rate as the cup and the car.  The force causing the water's acceleration is provided exclusively by the cup's rear wall.  The wall's acceleration/Action force is an external (contact) force impressed against the water's vertical surface.  The higher pressure within the water nearest the cup's rear wall results in, and is balanced by, an increase in the water's height at the rear of the cup.  Meanwhile the water's surface to the front of the cup is reduced due to the reactionary shift of water to the rear due to being accelerated by an external (contact) force.  Recognize here that if the cup is much taller than before, the water will simple be forced to rise up the cup's tall wall with none of the water escaping the acceleration/Action force by slipping free over the rim.

    Can we agree that it takes the application of a horizontal acceleration/Action force to cause horizontal acceleration for the water in the glass?  When water does rise high enough to reach the rim due to the pressure of acceleration, what happens to the water that slips over the rim?  As it escapes the accelerating cup, it also escapes the effect of the cup's acceleration/Action force.  Thus by all accounts horizontal acceleration ceases for the water as it escapes over the cup's rim.  To conclude that the escaping water ceases to accelerate due to the presence of the water's "inertial" "tendency" to "resist" acceleration is just plain silly.  The escaping water has no option but to cease horizontal acceleration the moment it slips over the accelerating cup's rim.  This is rest-motion behavior represents the water's default state of motion that automatically occurs when the event-causing acceleration/Action force becomes absent.  To apply the word "tendency" here is a mistake.  The escaping water has no other choice but to return to the default state of rest-motion in the horizontal direction when the cup's acceleration/Action force becomes absent.  To apply the term "resist" is a mistake.  The water in the cup that continues to experience acceleration does nothing to "resist" that acceleration.  It simply offers up a supporting acceleration/Reaction force that is caused by the action force responsible for the water's acceleration.  No acceleration reducing or slowing here.  Just 100% support for the cup's acceleration/Action force in complete agreement with Newton's LAW I.  To apply the term "inertia" to the escaping water's automatic return to rest-motion is also a mistake.  This cancellation-of-acceleration event, automatically occurs when the acceleration/Action force no longer applies to the escaping water.  No invention of an imaginary "inertia" is needed to explain the escaping water's sudden loss of acceleration.  The acceleration/Action force went missing.  So too went the water's acceleration.  "Inertial", "tendency", and "resist" are three words than have no meaning in accelerational events.  Use of any of these three non-terms by a science author are strong clues as to his or her degree of understanding.  It is important to avoid the use of all three when describing Physical events.  More importantly, once again I see no valid role here for the Newton's misunderstanding known as "inertia", nor do I see a valid role anywhere else within the many and varied events of this Universe. 

    Conclusion
      Isaac Newton's "inertia" simply does not exist.  In short, Newton's "inertia" is imaginary.  While Newton did make an effort to define his misunderstanding of "inertia", I find it curious that seldom if ever did he use "inertia" as a component in the explanation of the hundreds of events he described in his various works.  Perhaps he included his "inertia" definition in PRINCIPIA due to his feeling pressured by a general expectation as to a "cause" for an object's uniform motion.  That might explain Newton's ironic choice of words when he referred to "inertia" as the "force of inactivity" as if he thought a forceful "cause" was needed for the maintenance of a uniformly moving object's causeless default state of inactive rest-motion.     

Is Galileo Galilei the Father of Inertia?
     Physics authors often refer to Galileo as originating the concept of inertia. Curiously I have found no mention of any such concept within Galileo's many works. What I have found is that Galileo clearly recognized that in the absence of an acceleration/Action force, an object has no choice but to continue on with its motion unchanged. In the words of Professor Stillman Drake, a noted authority on the life and work of Galileo, "Galileo never asserted any vis inertiae or "force of inactivity" as Newton did." In his discussion of the issue, Drake went on to say "Many blame Galileo for his inability to invent inertia-..." Now that we see that "inertia" does not exist, we are free to recognize it is to Galileo's credit that he had no part in imaginary "inertia's" invention [3].

Aristotle's "Mover" Reborn As Newton's "Inertia"
     To Aristotle, a uniformly moving object's motion was maintained by the presence of a "mover". But Galileo revealed that since it takes a force to change a uniformly moving object's motion, when such a force is absent, no change can possibly occur to the object's uniform motion. No "mover" required. Then Newton wrote in Definition IV that "a body maintains every new state" (of motion) "it acquires by its inertia only." Here Newton effectively voided Galileo's logical understanding of an object's uniform motion, as being a causeless default state of motion. Instead Newton's "inertia", like Aristotle's "mover" gave the general audience something acceptable to fill the role of "maintaining" an object's uniform motion when no such role existed.

A Cause For Uniform Motion Is Expected
     For 16 centuries, Aristotle's "mover" satisfied the general need for a cause to explain uniform motion. Clearly, Galileo's causeless understanding of uniform motion failed to satisfy this need. But then Newton's "inertia" cause was readily accepted as representing the expected answer. Credit goes to Galileo for recognizing the causeless truth. With the demise of Aristotle's "mover" and now Newton's "inertia", can we resist inventing yet a third imaginary "cause" solely to satisfy the illogical but enduring general need for one?

    If Stillman Drake were alive today surely he would be relieved to learn that, to Galileo's credit, "inertia" never was in need of "invention".

Ethan Skyler
July 4, 2003

P.S.  You are invited to join in the ongoing adventure into the realm of Classical Physics available on the Internet at http://www.Universal Physics.org.  At the Universal Physics Journal, the goal is to lay the foundation of Universal Physics wherein Galileo's and Newton's many absolute concepts will once again rule the science.

References
[1] Sir Isaac Newton, 1686, 1729, Mathematical Principles of Natural Philosophy and His System of the World, 1934, 1962, PRINCIPIA, University of California Press, Berkeley, Los Angeles, London, page 2 - 13

.[2] The accelerational force terms acceleration/Action (a/A) and acceleration/Reaction (a/R) were developed by Ethan Skyler and Ryan E. Skyler to aid in the recognition of the differing roles forces play during accelerational events.

[3] Galileo Galilei, 1638, Two New Sciences Including Centers of Gravity and Force of Percussion, Translated, with a New Introduction and Notes, by Stillman Drake, 2nd Edition, Wall & Thompson, Toronto, page xxxii.

(C) Copyright 2003 - 2004 Ethan Skyler All rights reserved.

 

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