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If I've read Newton's Laws of Motion correctly in the Principia, it seems that Newton attributed the "change in motion" (momentum) to the "impressed force". Mathematically this would be read as $\Delta p \propto F$, right? But then how did it end up as $\frac{\Delta p}{\Delta t} \propto F$?

Also, I've read in other forums that Newton's Second Law is based on Galileo's experiments on falling bodies, where he treated acceleration and mass as distinct parameters. For example, his experiment with comparing free-fall of different masses yielded something along the lines of $\frac{F_1}{F_2} \propto \frac{m_1}{m_2}$ ($\propto a$). This threw me off even further, making me question how (and why) the heck Newton ended up with $\Delta p \propto F$ as his Second Law.

I'm not questioning its validity, I just want to understand how it came to be understood as $\frac{\Delta p}{\Delta t} \propto F$.

Qmechanic
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chevestong
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3 Answers3

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Newton didn't say "change in momentum", he said "alteration in momentum", and whichever he said, this means clearly, and with no room for doubt, rate of change of momentum, the limit of small $\Delta t$ of $\Delta P \over \Delta t$. This was understood this way by everyone who read the book, there is no way to misinterpret if you follow the mathematical things.

The experiments of Galileo showed that bodies in gravity have the same acceleration. This means that the Earth is imparting changes in velocity to particles. The notion of "force" is already present to some extent in the theory of statics developed by Archimedes, and gravity produces a steady force in a static situtation, and this force is proportional to the mass. If you know force is proportional to the mass, and the acceleration is the same for all bodies, it is no leap to conclude that a force produces a steady acceleration inversely proportional to the mass.

The second law was not the major innovation in Newton, this was known to Hooke and Halley and Huygens for sure. Newton's innovation is the third law, and the system of the world, the special problems.

Ron Maimon
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  • So in other words, Newton's statement $\frac{\Delta p}{\Delta t} \propto F$ was "derived" from Galileo's treatment? – chevestong Aug 24 '12 at 07:50
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    @ProSteve037: Yes, but there is no reason to put "derived" in quotes--- it is the natural mathematical statement of the fact that force produces acceleration. I don't know why you consider the mathematical statement somehow profound or confusing, it is obvious and straightforward, and was known to everybody by the mid 1600s. This is not Newton's discovery, his discoveries are more involved, like the oblateness of the Earth, the wobble of the moon, the cause of the precession of the equinoxes and so forth. – Ron Maimon Aug 24 '12 at 14:18
  • I've been shortchanging Galileo and Newton this whole time :-\ I had no idea that all this was known prior to Newton; university physics texts really like portraying Newton as the one who'd made these discoveries. But I think another thing that was confusing me was how the concept of weight was viewed during Galileo's time. I generally assumed that at that time weight wasn't viewed as a force but just some property of objects that compared their sizes or amounts. – chevestong Aug 26 '12 at 17:23
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    @ProSteve037: Physics books basically make up history, in a series of outrageous fabrications, and pretent that what is confusing to students is what was confusing to people back in the 16th century. Galileo didn't really discriminate in a precise way between mass and weight, at least not as clearly as Newton does, so maybe Galileo would get a question or two wrong on a modern test. But the concept of force is known from Archimedes law of the lever--- the force times distance must balance for a lever to balance. I don't see this confusion as particularly important historically. – Ron Maimon Aug 26 '12 at 22:16
  • I should've figured as much, I'd always read that Newton acted as the primary figure in the mathematical formulation of these concepts but I never knew how. Another thing that I was curious about was how Newton stated the 2nd Law as an axiom. I haven't fully read the Principia but wouldn't that mean that he's stating it as fact without proof? Or I guess you could surmise that he did so because it was so self-evident at that time, as you said. – chevestong Aug 27 '12 at 21:51
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    @ProSteve037: The way Newton was the primary figure is that he made a _respectable science+ out of Cavalieri's infinitesimal calculus, Barrows fundemental theorem, and Galileo's relativity and acceleration laws--- he showed the true scope of these discoveries, that they unlock the secrets of motion and dynamics, and give a true model of the solar system from simple first principles of Newtonian gravitation. This is an astonishing achievement, but it isn't the discovery of "F=ma", that was just known to everybody in 1666, the same way everyone knows perturbative string theory today. – Ron Maimon Aug 28 '12 at 02:04
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    Regarding the F=ma, Newton takes it as a new definition of force, which subsumes and extends the old one from statics. When things aren't moving the sum of the forces is zero and the sum of the torques is zero, and this reproduces Archimedes' law of the lever and bouyancy. So it's the same thing, but now it works for dynamics. The main discovery is how to calculate using continuous quantities like acceleration and velocity, and this discovery is modern calculus. Newton didn't choose to make a new language, he snobbishly preferred to sound like the ancients, so he fairly got bitten by Leibnitz. – Ron Maimon Aug 28 '12 at 02:09
  • Thanks so much Ron, really. This helped a lot :] I have to ask, though; could you recommend any reads/books for me? (now that you have a general sense of the history that I'm interested in) I don't mind going as far back as Archimedes if I have to haha. The journey of scientific discovery and thought from Archimedes' Law of the Lever to Newton's Laws fascinates me and I would love to rectify any remaining curiosities on the matter. – chevestong Aug 30 '12 at 02:49
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    @ProSteve037: It's not hard--- the works of Archimedes are widely reprinted, I read the version in "Great Books of the Western World" vol 11 (one of only two volumes worth reading, the other is 18-19th century scientists, the rest of the series is crap), Euclid Appolonius Archimedes (it's in every high school library, as part of getting kids excited about great books). Galileo's "Dialogue" and "Two New Sciences" are essential, and Barbour's "Absolute or Relative Motion" fills in the other folks from the era. That, plus Newton's Principia (after learning the modern thing) is all I read. – Ron Maimon Aug 30 '12 at 04:32
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Isaac Newton, Philosophiae Naturalis Principia Mathematica, Axiomata, sive Leges Motus [1]:

Lex. II.
Mutationem motus proportionalem esse vi motrici impressæ, 
& fieri secundum lineam rectam qua vis illa imprimitur.
[...]

Mutationem: means change, alteration, variation, mutation, ... Motus: momentum.

So Mutationem motus means exactly the variation of momentum: $$ \frac{\Delta p}{\Delta t} $$

[1] http://la.wikisource.org/wiki/Philosophiae_Naturalis_Principia_Mathematica

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I will try to explain:

Let us go back to 1600s when force is not defined quantitatively, and try to understand as to how we can arrive to the definition.

Force: a feel of push or pull is named by humans as "Force".

Lets imagine one situation in which we have a toy gun which is pushing a small marble kept on an ice frame.

The following things happened: 1. Now the toy gun pushed the marble, using single arrow. 2. We plotted the instantaneous rate of change of momentum for the object on y axis and on x axis, we plotted Force. Now mind here that the force is not defined quantitatively yet; But intuitively, we can say that the toy arrow is applying a push on the marble (thus its applying a force on the marble).

Therefore on x axis, we plotted force as using one toy shoot.

Now, we used two toy arrows at the very same time and applied onto the marble.

Again we plotted rate of change of momentum on y axis, and on x axis, we can intuitively think that if ine toy arrow can provide some value of force, now two toy arrows will be providing twice that amount. Therefore, we plotted that on x axis.

We did similar experiments with 3 toy arrows, 4 toy arrows etc and plotted rate of change of momentum vs this pushing force on y-x axis respectively.

When we joined all these, for the marble of constant mass, it formed a straight line, passing through zero.

Thus one thing is clear that the force acting on a body is directly proportional to the rate of change of momentum.

F directly proportional to d(mv)/dt

Now to eliminate the constant of proportionality, we defined 'Newton' as a unit of force such that when we apply a force of 1 N, its capable of producing the acceleration of 1m/s2 for a body of 1kg mass.

If on the other hand, lets say we defined the unit of force as Ron, and defined that one ron is that force which will produce an acceleration of 0.5 m/s2 for a mass of 1kg, then we can write F=2ma.

But for the time being till, unit of force is Newton, lets enjoy with the following equation F=ma

lancer
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