L12 gap-filling test for 2R at KU (2012/01/19) prepared by Kow Kuroda
The script below was a transcription ofThe Feynman Lectures on Physics, Volume 1, Chapter 4created by Kow Kuroda, based on (http://www.amazon.co.jp/Feynman-Lectures-Physics-5-6/
dp/0738202835/ref=sr_1_8?ie=UTF8&qid=1326870123&sr=8-8).
Richard P. Feynman:
Conservation of Energy
0.1 Preamble
The Feynman Lectures on Physics: This lecture was presented by Dr. Richard Feyn- man on October 6th, 1961 at the California Institute of Technology. Volume 1, Chapter 4:
Conservation of Energy 0.2 Introduction
There are— will be no summary of the pre- vious lecture, huh. There’s no 1. summary of the previous lecture. You just to remember whatever you can remember from it. No vital points.
The, uh next two lectures after today will be given by Professor Matt Sands because I’m going to a meeting in, uh, Brussels. So, I’ll come back, uh next Tuesday, (and) I mean the Tuesday following.
0.3 Section 4.1: What is energy?
Today’s lecture is on, uh, one of the laws of physics, and, uh, beginning, of course, t uh, detailed looking at the different aspects of physics. We just finished, uh, description of, uh, things in 2. general and now we look more specifically, in particular, what a law of
physics looks like. And so I picked one out uh/as to illustrate the ideas and a kind of rea- soning that might be used in, say, theoretical physics. So, the lecture today is on the con- servation of energy.
There is a fact, or if you wanna call it a law, governing all natural phenomena that’s known today there’s no exception known to this. It’s exact as far as we know. And the 3. law’s called the conservation of energy. It states:
there is a certain quantity, and which we call energy, that doesn’t change in, uh, manifold changes which nature undergoes.
Now that’s a very abstract idea because it’s a mathematical principle. It says there’s a numerical quantity that doesn’t change when something happens. It’s not a description of a mechanism, or anything. It’s just a strange fact that you can calculate some number and when you all finish watching 4. nature go through her tricks and calculate a number again, and it’s the same. Something like the bishop is on the red square now, and after a number of moves, details are unknown, it’s still on some red square. It’s a law of this na- ture.
Since it’s an abstract idea, I would like to il- 1
lustrate the meaning of it with a lot of pecu—
uh silly example(s) but it does illustate the idea.
I want you to imagine a child, perhaps De- nis the Manis, who has blocks. But you must appreciate that these blocks are absolutely in- destructible and cannot divided into pieces.
They’re just— each is the same as the oth- ers. (And) I will suppose that he has, say, 28 blocks.
Now, this child is 5. put into a room by his mother, at/and the beginning of the day with these 28 blocks. At the end of the day, I don’t know why she’s so curious but she counts the blocks very carefully and discovers a phenemenal law that no matter what he does with the blocks, there are always 28 blocks.
So, this goes on for a number of days, until one day there are 27 blocks, but uh a little 6. looking around shows that there’s some under the log. You have to be careful to make it sure that you looked everywhere in order to make it sure that the number of blocks doens’t change.
One day, however, the number of blocks appears to change. There’re only 26 blocks.
7. Careful investigation indicates that the window was open and now looking outside, you can find other two blocks. So, you bring them back in again and everything’s alright.
Another day, careful 8. count indicates the existence of 30 blocks. (Laughter) This causes a considerable concination until it was realized that Bruce came to visit and he owned blocks and left perhaps a few.
So, after you get rid of some of these things, we close the window, we don’t 9. let Bruce
in then we see everything is going along al- right until one time we count 25 blocks.
But there is a box in the room— the toy box that the boy has and the mother is going to open the toy box but the boy says, “No, don’t open my toy box” and screams. She doesn’t lumped um uh— The mother was not allowed to open the toy box.
Being, however, extremely curious and somewhat 10. ingenious , she invents a scheme: she knows the block weights three ounces. So, she weights the box when they—
at the time she sees the 28 blocks, and weights 16 ounces, say. So, the next time when she wants to check she weight[s] the box again, subtract[s] 16 ounces and divide[s] it by 3. So, she 11. discovers the following: number of blocks in— that you can see that you have to add to this number of blocks seen, plus weight of box minus 16 ounces divided by 3 ounces and then it’s constant at every time, up to a point. There are then become some gradual 12. deviations , there appear some deviations but careful study indicates that the dirty wa- ter that’s in the bathtub is changing its level.
In other words, the child was throwing blocks into the water. You can’t see it because it’s so dirty but you can find out how many blocks in the water by adding another term to this formula, which is the height of water in tub 13. minus 6 inches divided by a quarter of an inch because each block, it goes— it’s a quarter of an inch.
And so, with the gradual increase in the complexity of the world, you’ll find out the whole series of terms representing the ways of calculating how many 14. blocks in places 2
where you’re not allowed to look. And as the result of this, it’s possible to find a complex formula— a quantity which has to be com- puted, which always stays the same in this sit- uation.
Now, the anal— I would now describe the analogue of this to the conservation of en- ergy. The most remarkable 15. aspect of it is that I want to abstract from this picture:
first, there are no blocks. You never see the blocks. So, you take away that term. Then you discover yourself calculating more or less abstract things. The analogue has the follow- ing points in it: first, sometimes, when you’re 16. figuring out the energy, some of it leaves the system and it goes away, or sometimes some comes in. And so, in order to verify the conser— vation of energy, you’ve got to be careful that you haven’t put any in and take any out.
Second, that the energy has a large number of different forms and it has a formula for ev- ery one of the different 17. forms , I’ve writ- ten a number of the forms here, I see, I left out the elastic energy and there isn’t formula for every one of these things if we find out what the formulas are in the total amount that wouldn’t change if you accept it going in and out from now— I’ll keep you that— well, we can do that now if you want.
So, that’s the relation of the 18. analogy . It is important you realize that in physics to- day, we do not have any knowledge of what energy is. My analogy is bad because we do not have a picture in physics today that en- ergy comes in little blobs of a definite amount.
It does not that way. and it is not de– def-
inite blocks but there are 19. formulas for calculating some numerical quantities when you added it all together, it gives 28: always the same. We have no deeper understand- ing of it than this. And that is an abstract thing: it doesn’t tell you the mechanism, or the 20. reasons for the various formulas.
調査
授業の方設計するために，以下の二つの点 に関して意見を述べてください．
(1) 問題の量は適切でしたか？
1. 多過ぎた
2. ちょっと多かった 3. ちょうどよかった 4. ちょっと少なかった 5. 少な過ぎた
(2) 聴き取る箇所の難易度は適切でしたか? 1. 難しいところが多すぎた
2. 難しいところが多かった 3. ちょうどよかった 4. 簡単なところが多かった 5. 簡単なところが多すぎた
他に意見があれば書いてくれてよいです．
今後の授業に生かします(成績には影響しま せん)．
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