# A few questions about different stuff (plus request for a good source to learn)

1. Jun 10, 2009

i am so confused.. does anybody know a good source to learn physics? every single source (book, website, etc.) says something different..

so.. can anybody please refer me to good reliable source?

here, for example one of the many things that confused me..
can i say that a *body* does work upon another body? and when it does, it loses energy, and the body that the work was done upon, gets energy?
that seems like absolute ******** to me, but it was actually written in one of my book..

oh, and in one website, they say "the normal force never does work, because it's always vertical to the motion".. well how about elevator?

and in my class we have learned that the only conservative forces are, mg and kx..
well, let say there is a constant force F (wind for example).
constant size, constant direction, won't it be a conservative force?
so we must keep looking for conservative force all the time, don't we?

thanks very much

2. Jun 10, 2009

### Staff: Mentor

A textbook would be the best source, but you may find it has a lot of math in it. There really is no way around this.
What is wrong with that? It looks fine to me. We can help you understand these things, but only if you tell us what bothers you about them.
"Normal" referrs to the direction of the force with respect to the direction of motion. So the force applied by an elevator cable is not "normal" to the direction of motion.
I'd need a little more context to that to understand the point, but forces come in pairs, with something applying a force to something else, which applies the same force back.

Wind is not a force, it is a motion of air. And being conserved doesn't have anything to do with time.

3. Jun 10, 2009

### diazona

What kind of physics?
If by "high math" you're talking about calculus, vectors, or linear algebra, there's only a limited amount you can do in physics without those...

A normal force is perpendicular to the motion, not necessarily vertical...

Nah, there are plenty of conservative forces. In fact, at the subatomic level, all forces are conservative.

4. Jun 10, 2009

### Staff: Mentor

They should have said "perpendicular" instead of "vertical." That's one problem with trying to learn physics from Web sites... you run into errors or poor choices of words. In this case I suspect the author's native language might not be English. Try to stick with web sites that are based at universities (lecture notes for courses, etc.), unless you have information from a good source that a certain "private" site is OK.

One site that I think most people here would recommend as a good overview or summary of basic physics is the Hyperphysics site at Georgia State University:

http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html

5. Jun 10, 2009

### maverick_starstrider

Physics IS applied math. That's life. In fact, I'm one of those people that believes that trying to explain physics in words only creates problems. 'Tis far better to try and make the math understandable and present the physics as it actually is. Then you get none of this:

entropy = disorder
space-time = a linen sheet with a bowling ball on it
string theory = a crazy dancing string at every point of space
big bang = this huge explosion where all the mass in the universe explodes outward through space

and et cetera.

6. Jun 11, 2009

### granpa

7. Jun 11, 2009

### clem

"to learn physics...with no high math"
That's your problem. Physics needs math. How high depends on what you want to learn.

8. Jun 15, 2009

thanks for all the replies
as i am sure you've notice, english isn't my native tongue so some of the misunderstanding might have occurred because of this

so first of all, do not misinterpret me, i LOVE math, i just don't know calculus, and stuff like that..

any recommendation?
well..first of all i thought only a force can be said to "does work"

secondly, i might got confused, but.. if some body, for example, moving on a surface, it's speed is decreasing, so it loosing it's kinetic energy, while a force has been acted on it, right?

i meant this force
http://en.wikipedia.org/wiki/Normal_force
isn't this the force that make a person in an elevator to go up?
haha by "we must keep looking for conservative forces all the time"
i meant, while soving a problem, we, as a solvers, should keep thinking wheather any of the forces is a conservative or not , and we must do that all the time :P
(we might get a wrong answer, if we don't pay attention to that, right?)

about the wind force, does it really matters? can't i just assume there is a force that direct to the right for example, that act on every body around?
i know that it can't happen in nature, but just as an exercise, is it OK?
well too bad.. i guess i'll learn it personally then :P
what kind of physics? err i don't know.. this kind.. mechanics..(?)
well, actually i have no idea what the diffrence is :D

:O
is that so? how come?
well as i said, i am not an english speaker,
and actually this specific book wasn't in english, so this is actually my fault, by mistranslate it.
and thanks for the site
oh i completely agree!
and i do as someone that barely know physics at all, but i've already guessed that this is the situation.. physics can't be explained without math..
all those pretentious TV programs about physics..
i was sure that you all physicist are looking at those programs open mouthed and just think "what on earth are they talking about"
haha.. well nice to meet someone that fit my modele
anyway, i defintely havn't tried to escape math, no way.
i somewhy thought everything in this level can be explain fairly without calculus etc.

but it all seemed very drab to me (dunno whether drab is what i exactly mean)
it just one formula after another, no explantion what the logic behind things..
physics is fascinating :D i'll learn how much it will take!

9. Jun 15, 2009

### diazona

Well... yeah, I guess technically that is right. When we say that body #1 does work on body #2, what we really mean is that body #1 exerts a force on body #2, and it's that force that does the work.

That sounds correct. If a body's speed is decreasing, it is losing kinetic energy, and that means that some force is acting on it. The force would be doing negative work in this case (since the kinetic energy is decreasing).

ohhhh... I see what you mean Yes, the force that makes the person inside an elevator go up is a normal force.

I wrote something incorrect in my last post when I said that the normal force is always perpendicular to the motion. I should have said that the normal force is perpendicular to a surface, not perpendicular to the motion. (I was thinking of a block sliding along a table, where the normal force is perpendicular to the motion) Whenever two objects are in contact along some surface, and each one exerts a force on the other perpendicular to the surface, that force is called the normal force. It's also possible for each body to exert a force on the other which is parallel to the surface - friction for example - but that is not a normal force.

Well, I don't think it's always necessary to know whether the force is conservative or not. It depends on the problem.

well too bad.. i guess i'll learn it personally then :P
what kind of physics? err i don't know.. this kind.. mechanics..(?)

http://en.wikipedia.org/wiki/Perpendicular
http://en.wikipedia.org/wiki/Vertical
Vertical always means "oriented in the up-down direction" but perpendicular refers to two things that are at right angles. In your elevator example, the force which pushes the person upward is vertical, but the floor of the elevator is horizontal. So the force is perpendicular to the floor (and that's why it's a normal force ).

well... this explanation may not satisfy you, but here it is anyway: the definition of a "conservative force" is a force that is "path independent." Here's what that means: imagine that you have an object subject to some force. If you move the object from point A to point B, the force does some work on the object. Then if you move the object from point B back to point A, the force again does some work on the object. If the force is conservative, the work done going from A to B exactly cancels out the work done going from B to A - even if you take a different path in reverse - so by the time the object has completed its round trip, it has exactly the same amount of energy it had before it started. That's only true for conservative forces. For example, you have a brick sitting on the floor. If you pick it up, walk around with it, and then go put it back down in the same spot, it has the same amount of energy it started with, because gravity is conservative.

Every conservative force has a corresponding potential energy. You may have heard of gravitational potential energy,*
$$U = mgh$$
(that's mass times gravitational acceleration times height). There is also electrical potential energy,
$$U = k\frac{q_1 q_2}{r}$$
and many other kinds. These formulas for potential energy wouldn't exist if the force weren't conservative. (Think of friction as an example of a nonconservative force - it has no potential energy associated with it)

Well, when physicists are coming up with theories to describe the way subatomic particles behave, they much prefer to come up with formulas for the potential energy, not the force itself. So the first thing to do when describing a new force is to figure out what potential it corresponds to. And so far, every force that has ever been discovered acting on a subatomic particle has a corresponding potential energy. If there were nonconservative forces acting at the subatomic level, physicists would be unable to come up with a potential energy formula for them - and in fact, they could do experiments like the path independence thing I described a couple paragraphs back to prove that it was nonconservative. But that has never happened.

You might be asking yourself how nonconservative forces like friction can exist at all, if all subatomic forces are conservative. Well, actually friction turns work into heat (actually "thermal energy"), which is the random vibrations of atoms and molecules. If we could analyze the motion of all those atoms and molecules individually, and keep track of all the little bits of energy passed between them, friction would look like the total effect of many tiny conservative forces. But there are far too many atoms and molecules to keep track of them individually, and when we do that, friction seems to be just draining energy from the object it acts on, which is a signature of a nonconservative force. (Sorry this last paragraph is not especially clear, but I can't think of a better explanation that doesn't involve a lot of higher-level physics)

*Disclaimer: $U=mgh$ is the gravitational potential energy in a constant gravitational field, which means that formula is only valid near the surface of the Earth.

10. Jun 18, 2009

ha?
i am sure i have replied here..
is it possible it has been deleted for some reason?

11. Aug 5, 2009