Mad_Eye said:
well..first of all i thought only a force can be said to "does work"
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.
Mad_Eye said:
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?
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).
Mad_Eye said:
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?
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.
Mad_Eye said:
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?)
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..(?)
Mad_Eye said:
well, actually i have no idea what the diffrence is :D
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
).
Mad_Eye said:
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.
