Name: 4. 牛頓定律（續）和斜面（4. Newton's Laws (cont.) and Inclined Planes）
Uploaded: 2022-06-26T18:14:27.000Z
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情態副詞

I would ideally like to have a class with maybe just ten

students so that we can have a lot of dialogue and I can find

out what you're thinking about, address your questions.

That'll make a lot of fun but that's not possible in an

introductory course. But I think you should feel

free to interrupt or ask questions or discuss any

reasonably related issue that I didn't get to.

for everybody because I told you many times the subject is not

new to me. It's probably--it's new to you.

So what makes it interesting for me is the fact that it's a

different class with a different set of people with different

your ability to surprise me is limitless because even on a

subject which I thought is firmly fixed in my mind,

some of you guys come up with some point of view or some

question that's always of interest to me,

of people who've been practicing this business for a

All right, so I think last time I got off on a rant about all

the different pedagogical techniques, some of which I

don't endorse fully. I'm going to go back now 300

years in time, to one of the greatest laws

that we have because--look at the power of this law,

and all the mechanical phenomena that we see,

around the world, can be understood with this

examples on how to put this law to work, because I think I at

least made you realize that simply writing down the law does

not give you a good feeling for how you actually use it.

So, maybe you have understood it, but I'm going to remind you

one more time on how you're supposed to use this law.

So, I'm going to take a concrete example.

The use of any law of physics is to be able to predict

something about the future, given something about the

can be categorized that way. So I'm going to take a very

simple problem, a problem to which I will

return in great detail later on; but let me first start

with--let me start with this problem, which I will do very

quickly now and we'll come back and do it more slowly later.

The problem looks like this. So, here is a table and here's

and let it go. So that's the knowledge of the

when I let it go, what's this guy going to do?

you can replace the spring by the Sun, which is attracting the

you can make it more and more complicated.

But they all boil down to a similar situation.

I know some information now and I want to be able to say what'll

when I want A and I want to know what'll it do next.

Remember, when we go back to the laws of Newton,

the laws of Newton only tell you this--and we've been talking

The first thing you have to know in order to use the laws of

Newton, you have to separately know the left-hand side.

You have to know what force is going to act on a body.

You cannot simply say, "Oh, I know the force on the

you got to have some other means of finding the force.

And in this case, the force on the mass is due to

various amounts and I see what force it exerts.

Now, I think you know now in practice how I know what force

it exerts, right? I pull it by some amount,

attach, say, the one kilogram standard mass.

I see what acceleration it experiences, and m times

that acceleration, or 1 times the acceleration,

is the force the spring exerts. So I pull it by various amounts

and I study the spring. And I've learned,

by studying the spring, that the force it exerts is

If I start off the mass in a position where the spring is

neither expanded nor contracted, that's what we like to call

pull it to any point x, that's the force the spring

independent study. People who work in spring

physics will study springs and they will find out from you,

find out and tell you that any time you buy a spring from me

it'll exert this force. And they have done--they

figured that out by pulling the spring and attaching it to

various entities and seeing what acceleration it produces.

There, the masses are taken to be known, because you can always

borrow the mass from the Bureau of Standards,

or we discussed last time how if you have an unknown mass you

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4. 牛頓定律（續）和斜面（4. Newton's Laws (cont.) and Inclined Planes）

constant

realize

positive

negative