Sir Isaac Newton led the foundation of his famous laws of motion during the 17th Century but at that time SI system hadn't existed. So in which units did he define force? Did he define it in some other units or was later redefined by scientists who made the SI system?

  • $\begingroup$ I believe one of his early efforts was to jump downwind as far as he could, and then upwind as far as he could. The difference between the two was a measure of the force of the wind. $\endgroup$
    – mmesser314
    Aug 22 '21 at 19:36
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    $\begingroup$ Would History of Science and Mathematics be a better home for this question? $\endgroup$
    – Qmechanic
    Aug 22 '21 at 19:42
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    $\begingroup$ It doesn't make any difference to the physics as to what units he used as long as they were used in a consistent way. $\endgroup$
    – StephenG
    Aug 22 '21 at 19:54
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    $\begingroup$ Considering how even today units for mass are used for force as well (implies multiplication by g) and the daily interchangeable use of weight and mass, probably Sir Isaac Newton just used whatever unit for mass/weight was used back then. I.e. pound and derivatives. $\endgroup$
    – fraxinus
    Aug 22 '21 at 21:07
  • $\begingroup$ @mmesser314 That would measure force per Newton. $\endgroup$
    – JEB
    Aug 22 '21 at 22:08

If you read the article On the Concept of Force: How Understanding its History can Improve Physics Teaching you will realise that Newton's ideas about force are not the same as those of today. Indeed Newton thought that there were different types of force.
No unit of force was proposed until 1873, when the dyne was introduced. Historically, forces were first quantitatively investigated in conditions of static equilibrium where several forces cancelled each other out. The strength of any force was expressed in terms of an equivalent gravitational attraction, with weights or masses measured in grains.


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