# Newton, Galileo, and Gravity

One of the few things I remember from my high school physics class is my teacher telling me that Newton discovered things like the universal law of gravitation simply because his initial premise was correct. Newton assumed that "things moving in a straight line tend to keep moving unless compelled to stop". Everyone before Newton basically failed because their initial premise "things moving in a straight line tend to slow down and stop and unless compelled to keep moving" was wrong and this list of failures include Galileo. Gelileo made this mistake and hence history crowns Newton for this achievement instead of Galileo even though Galileo was so close.

But Thomas Pornin's answer here says that Galileo made the correct assumption. So my questions is, did Galileo make the correct assumption or the wrong assumption? If he made the correct assumption, then what stopped him from figuring out the law of universal gravitation? Why stopped him from getting crowned before Newton? It would be great if the answer can be backed by some sort of historical evidence/references.

• I'm not sure this is a good question. I could extend it to ask "Why didn't Archimemdes make the correct assumption? What stopped him?" Science progresses- that's the nature of the beast. Galileo made certain assumptions- the full formalization of the 3 laws of motion came later. Sep 12, 2014 at 2:29
• Is this really a history question? Sep 12, 2014 at 16:57
• Going by the thread and your acceptance of the answer, I'd request you to modify the question to ask what scientific and mathematical points were not yet developed by Galileo which Newton went on to perfect. When I read your question "What stopped him" it sounded like why couldn't Galileo have done it himself- a subtle difference I agree- but as you may realize- subject to a different interpretation altogether. Sep 13, 2014 at 2:57

The principle of inertia

Galileo was an early and prominent advocate of the principle of inertia -- roughly stated, that things naturally keep moving rather than naturally slowing down. In his 1624 reply to Ingoli, he described a specific experiment, which he claimed to have carried out, in which a rock was dropped from the mast of a moving ship, and struck at the base of the mast rather than behind it. (In writings from this era, it's sometimes hard to tell what's a thought experiment and what's a real experiment.) His views on inertia were tied up with his advocacy of Copernicanism, which in turn was at least one major factor that got him in trouble with the Church. Galileo did not understand inertia in mathematical detail, and in particuler he did not clearly understand that it only applied to linear motion, not circular motion.

Gravitation

If he made the correct assumption, then what stopped him from figuring out the law of universal gravitation?

Galileo lived after Brahe and was a contemporary of Kepler. So Galileo had the data about planetary motion, but what he lacked was the necessary math (calculus) and physics (Newton's first and second laws). Without these ingredients, it wasn't possible for him to figure out, as Newton did, that an inverse square force law would explain the observed motion of the planets. Even much later, during Newton's lifetime, the notion of an inverse square law was in the air and widely suspected, but only Newton had the tools to knit everything together.

• Just a clarification, you said Galileo was lacking Newton's first and second law. But isn't the principle of inertia the first law or am I missing something? Sep 12, 2014 at 19:48

Newton assumed that "things moving in a straight line tend to keep moving unless compelled to stop"

This is his first law as stated in Newtons Principia in 1666; but 20 years earlier, during the English Civil War, Hobbes wrote in his Leviathan:

that when a thing lies still, unless somewhat else stir it, it will lie still forever, is a truth that no man doubts.

This was actually a truth first established by Aristotle; but only on Earth; in the heavens he assumed that the natural motion was not rest but circular motion. Hobbes however goes on to say:

But [the proposition] that when a thing is in motion it will eternally be in motion unless somewhat else stay it, though the reason be the same (namely that nothing can change itself), is not so easily assented to.

Its also known that Newton read Lucretious's epic cosmological poem De Rerum Natura on the Epicurean atomic theory of matter:

The opening exposition of book 2 descends into the details of atoms' behaviour and qualities. They are in perpetual motion at enormous speed, since in the void they get no resistance from the medium, and when they collide they can only be deflected, not halted.

Thus it is friction (collison in his words) that slows moving atoms about; its worth pointing out just how close he was to the 19C atomic theory of matter as developed by Boyle & Dalton:

Their weight gives them an inherent tendency to move downwards, but collisions can divert those motions in other directions. The result is that, when in a cosmic arrangement, atoms build up complex and relatively stable patterns of motion, which at the macroscopic level appear to us as states of rest or relatively gentle motion.

It was Gassendi round about the same time that Hobbes was writing who made atomism respectable again in early modern Europe:

The essential feature of atoms which does the most work in Gassendi's physics...is their inherent weight, which gives them an intrinsic, natural tendency to move.

And he developed the notion of constant motion:

Given this tendency, atomic rest is either provisionary or else an illusion. Atomic weight gives rise not only to a simple capacity for constant motion, but also to a range of more complex behaviors:

Its also worth noting that Aristotle had a theory of gravity - the natural motion of bodies; though of course he didn't call it by that name; its Newtons achievement to universalise that phenomena; he breached the division between the celestial & terrestial sphere - thus 'universal gravity'.