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Notes on 'The character of physical law, 1, the law of gravitation', a lecture by Richard Feynman, part of a series.   Share:  
Thrust of argument: This is just my rough notes, trying to very broadly bullet point the entire specific lecture. Direction of resistance / implied resistance: I have watched it many times and I want to now move to a new level of consolidating the information being learned.

 

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Removal of resistance: Here we go. Unification: - we respect the arts more than the sciences (subject of joke)

- renaissance artists' main concern, for man - we the scientists want to concern ourselves with a wee bit more than that

- patterns and rhythm, only apparent to the eye of analysis, not the eye alone - physical law is these rhythms and patterns

- the most overall general qualities of nature

- feynman wants to not speak in generalities, he wants instead to be understood 'in an honest way rather than a vague way' so he says he'll try to give instead of generalities, an example of physical law, so that we have at least one example of what he's speaking about generally

- he's going to tell us about the theory of gravitation, the phenomenon of gravitation, the law of gravitation - gravitation, in a word

- he says it is an excellent example of what modern science's character is

- "I'm more interested not so much in the human mind as in the marvel of nature who can obey such an elegant and simple law as this law of gravitation, so our main concentration will not be on how clever we are to have found it all out but on how clever she is to pay attention to it."

- 'the law is that two bodies, or bodies exert a force upon each other, which is inversely as the square of the distance between them and varies directly as the product of their masses'*

- 'a body reacts to a force by accelerating or by changing its velocity every second to an extent inversely as its mass (it changes velocity more if its mass is lower, and so on)'

- everything else about the law of gravity is a consequence of these two remarks, although it requires mathematical knowledge to see all of the consequences

- the ancients first concluded that the planets and earth went around the sun

- this discovery was made again by Copernicus, later, after everyone had forgotten it had already been made

- what was the motion?

- Tycho Brahe, a rich man, set up an experiment: noting down the positions of the planets in the sky at various times, and thus distinguish the alternative theories from one another

- 'this is the key of modern science and the beginning of the true understanding of nature, this idea: that to look at the thing, to record the details and to hope that in the information thus obtained may lie a clue to one or another of a possible theoretical interpretation.'

- 'it's only through such hard work that we can find out anything'

- The data then came into the hands of Kepler

- He thought they went around the sun in circles with the sun off centre and noticed one planet was 8 minutes of arc off, and he felt this was too big for Tyco Brahe to have made an error, and it was not the right answer. Because of the precision of experiments he was able to proceed and find: 1. the planets went in ellipses with the sun at the focus, 2. equal areas are swept in equal times (meaning they go faster when they're closer to the sun and slower when further away), 3. the times it takes the planet to go around are related to the size of the orbit; the times go as the square root of the cube of the size of the orbit (the biggest distance on the ellipse)

- that is a very complete description of the motion of the planets around the sun

- 'meanwhile galileo did a number of experiments which led him to discover the principle of inertia: that if an object has nothing acting on it and is going along at a certain velocity in a straight line, it will go with the same velocity in exactly the same straight line forever'

- newton then asked 'when it doesn't go in a straight line, then what?' - he answered that a force is needed to change the velocity in any manner;

- if you are pushing it in the direction that it moves, it will speed up

- if you find that it changes direction, then the force must have been sideways

- the force can be measured by the product of two effects: how fast/much does the velocity change during a small interval of time and the mass

- the force is the acceleration multiplied by the mass

- the reason why things coast forever has never been found out: the law of inertia has no known origin

- newton was able to demonstrate that the statement that equal areas are swept in equal times was a direct consequence of the simple idea that all of the changes in velocity are directed exactly to the sun, even in the elliptical case

- 'from knowing how the periods of the different planets vary with the distance away from the sun it's possible to determine how that force must weaken at different distances and he was able to determine that the force must vary inversely as the square of the distance'

- this was nothing new - only 2 things - which Kepler said in a different language

- the next statement was to generalise this and say that every object attracts every other object

- how far the moon falls in one second = 1/20th of an inch

- we're 4000 miles away from the earth's centre and the moon is 240,000 miles away

- so if the law of inverse square is right, an object at the earth's surface should fall in one second by 1/20th of an inch times 3600, because the force has been weakened by 60 x 60, which is 16 feet in one second

- it was known from galileo's measurements that things on the earth's surface fell by 16 feet in one second

- newton was able to calculate what the shape of the orbit would be and found that it would be an ellipse

- the tides now had an obvious explanation (the pull of the moon on the earth)





[to be continued][heavy stuff]
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*Newton's Law of Gravitation:

 

 

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Click here to read about Shams Pirani, the editor and chief author on this grid - note, if you can actually prove anything written above wrong, I would gladly, if the proof is sufficient, correct what I've written and what I think - if I could, however, prove your attempted proof wrong, then I would accordingly say so and maintain whatever point of view is completely based on fact and proof.