A Letter From Isaac Newton

August 4, 2004


Donald R. Kanner

This SMART website is hosted by the Illinois Institute of Technology

4 August, 2004

Dear Teachers of Natural Philosophy,

    Whilst I am mildly amused by your modification of the name of my field of study to the Physics, I must admit that I am rolling over in my grave with momentous questions regarding the methods by which you teach my discoveries!

    Why do you refer to the equation F = ma  as my Second Law?  I never wrote this equation!   In my Philosophiae Naturalis Principia Mathematica.   I stated my Second Law as follows: 

    The change of motion is proportional to the motive force impressed; and is made in the direction of the right line in which that force is impressed.      (Thank you Florian Cajori, for your excellent English translation of my Latin.)

    Given that I defined the term motion as you now define momentum, and adding more modern terminology, you should be teaching my Second Law as follows:   

    Change of momentum is proportional to the net force impressed and is in a straight line in the direction of the impressed net force.

    This is a great starting point.  You can then go on to inquire of your students on the need for the passage of time for a change to occur.  The result is the fact that impulse, a.k.a. force x time, is equal to change in momentum.  Frankly, I really hate it when those physics text authors fail to point out that the equation Ft = (delta)mv is actually my Second Law adjusted for the time required for the change to take place!  From this equation, it is easy to show your students how you physics youngsters developed the equation F = ma.  

    Secondly, I move on to the combined topics of my Third Law and circular motion .  For some reason authors of textbooks on the physics fail to realize that centripetal forces come in action-reaction pairs!  Why are centripetal forces so often presented as singular entities, as though my Third Law did not apply?    Surely, you must realize that a mass circling your hand on the end of a string is also applying a centripetal force to your hand.  Take a close look at your hand.  Surely you see that it is also moving in circles. The reaction force to your hand's centripetal force through the string upon the mass is the mass's inertia through the string upon your hand.  Both the mass and your hand circle around the system's center of mass.  Sadly, too few of you teach this.  Instead, too many of you open up a can of worms by mentioning the term centrifugal force, which even I know is a fictitious force!

   Thirdly, I move on to my First Law, The Law of Inertia.  How many of you have taken the time to read my Definition III, which is to be found on page 5 prior to the appearance of my First Law on page 14.  For those of you who have failed to read my text, Definition III follows below:

   The, vis insita, or innate force of matter, is a power of resisting, by which every body, as much as in it lies, continues in its present state, whether it be of rest or of moving uniformly forwards in a right line.

   In the explanation that follows I go on to tell you that this is to be called inertia, (vis inertiae), or force of inactivity.  In my next sentence I go on to say that this force only exists when another force is impressed upon a body.  You teach from textbooks that say that mass itself is the inertia.  Why do you not teach, as I have indicated, that reaction force is the inertia? 

   Should you have any reactions to the above or answers to my questions, please feel free to contact me through two media: 1. the internet, and  2. drkanner@cps.edu

    Most sincerely,

    Isaac Newton

    P.S.  Ruffles didn't invent ridges, I did!   Check out the edges of your larger denomination coins.,  I. N.



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