Large Hardon Collider

[Shiroikage]

Actually, yeah, hit me up with a PM too.
If you want, we can even trade publications so you can be assured that I am not trying to fuck with you.

[Eliseos]

Edit: The book (which is in my car and fuck the cold) gives instructions to integrate kxdx between xi and xf and just use the values of integration as variables and cancel. This pisses me off, not sure exactly why, but I feel it's a total cop-out regarding some inherent property to this type of problem that allows them to not explain the calculus to the students who haven't learned it yet. Am I wrong that if you don't know the limits of integration you just have to take an indefinite integral and not some made-up xi squiggle xf magicmath?

This happens quite a bit actually. We used it in thermo a lot, when calculating entropy. Since the integral of 1/x dx is ln x, it didn't really mean much to say (for example) ln volume. It made more sense to have some final and initial value so you could see how the integral changes if the final volume is smaller or larger than the initial volume, like ln (Volume_final/Volume_initial).

It also shows up in a lot of proofs. If you're proving something related to distance (like potential energy for example) you will always see proofs from point a to point b. In the potential example they'll show the line integral from a to b is path independent, and when a=b the total integral is zero.

Plus, if you don't add in limits of integration, you'll have to add an arbitrary constant to the result so it's easier to just plug in some initial and final values for the variables anyways.

[BerenTebogo]

I've never, ever seen that before. GRAR. Frustrating.

[Kaylia]

don't be hatin' on biochemistry

We physicist spend most of our day hating non physicist! On rare occurence, we might also attempt to solve a physics problem.

I wondered if anyone could help me. Science has always interested me but i've never really gone any further than having an interest. Looking for somewhere on the web where i could learn some pretty basic science, any decent sites for this kind of stuff?

What kind of sciences are you interested in? If you want to read popular sciences, there was many books posted on last page. If you're actually interested in solving simple physics problem, the best way to start would be to grab undergrade book and read them seriously.

[Shiroikage]

We physicist spend most of our day hating non physicist! On rare occurence, we might also attempt to solve a physics problem.

I am not so sure we spend our days hatin', but we certainly do have a tendency of spending our days being less productive than we could be as well.

[Kaylia]

Getting really frustrated with a physics problem. I've solved it, got the right answer, my friends have solved it, got the right answer. Problem is we've used two different methods which appear to be entirely irreconcilable and it's going to drive me nuts if I don't really understand the heart of the problem.

"A 1588 g mass is on a horizontal surface with mu-k = 0.400, and is in contact with a massless spring with a force constant of 745 N/m which is compressed. When the spring is released, it does 3.73 J of work on the mass while returning to its equilibrium position. Calculate the distance the spring was compressed."

Now, how I solved it was Workdone = 1/2*kx^2 or basically 3.73 = k/2 * x^2

How my friends solved it (i believe, nobody is on at the moment) 3.73J = (1/2)kx^2 - μmg*x and used the quadratic formula I believe.

Now I used my method on all of their problems (different masses and mu values) and produced the right answer for every distinct total work given, which leads me to believe that something funky is up. I think the root of the screwy-ness has to do with hooke's constant being given in joules, but I can't connect the dots 100%.

What about this problem makes the mass and coefficient of friction both relevant and irrelevant at the same time?

Edit: The book (which is in my car and fuck the cold) gives instructions to integrate kxdx between xi and xf and just use the values of integration as variables and cancel. This pisses me off, not sure exactly why, but I feel it's a total cop-out regarding some inherent property to this type of problem that allows them to not explain the calculus to the students who haven't learned it yet. Am I wrong that if you don't know the limits of integration you just have to take an indefinite integral and not some made-up xi squiggle xf magicmath?

First of all, the answers you posted can't be equal unless μmg*x is equals to 0 (and it make no sense in context). They probably did it differently if you got the same answer.

Anyway, while both method may look irreconcilable, try to put it this way. As long the spring is compressed, a force will be applied on the mass. Normally, the system would be at rest at x=0, but the friction you're using in the problem is never equal to zero. Because of this, the system won't be at equilibirum where it normally is.

Conceptually, this is problematic because at x=0, the friction will actually pull your object back and move it toward a different equilibrium (where f_spring + f_friction =0) . This different equilibrium is probably the reason why you have trouble understanding what happens.


About your second point, work represent the amount of energy required to move an object in space from a point xi to another point xf following a particular path. Using indefinite integrals in this case would make no sense, because the definition of work itself require a start and an end.

In its line integral form, work is given by:
http://upload.wikimedia.org/math/7/6...1089a9493b.png
Because your problem is linear (x axis only), you can rewrite it as a definite integral between xi and xf, and ignore the whole vectorial approach.

If you want to know the effective amount of works, you could add up every force and solve this integral.
http://img687.imageshack.us/img687/4800/worky.png
xf= 0 was set arbitrarily in this case, but we selected this value since it work well with Hook law (it's always a good idea to set one limit on the "zero potential")

[edit]
going to continue in another post

[Kaylia]

3.73 joules represent the amount of energy stored (and released) by the spring, not the total work done by the system. It doesnt matter if your friction force is 0, -5, 10, or infinity, the energy between the initial and final state will be the same. If your friction was actually very high (using mu*Fgrav), your spring would compress even more and store additional work! Of course, this make no sense in physics, but that's what happen when you use engineer equation to describe the world (sorry, couldn't pass that one!).

Anyway, since the energy of a spring is given by this (E= ½*k*x²), all you need is the potential energy and k to solve the problem, and they gave you both in the questions. The friction is never involved.


Like I said twice already, what the friction actually do is change the point of equilibrium. What your friends probably did (consciously or not) is find this new point of equilibrium, and from there, figure out a relation that allow them to find the real size of the spring. They reach the same result because somewhere, they probably substract the "work/energy" done by the friction.

[BerenTebogo]

4:30am wrestling with insomnia so it makes no sense to me at the moment, but what you're saying and the steps you're following look like they will make complete sense later. Thanks a bunch kaylia! I'll take a second look at it after I'm home from my morning classes.

[Eliseos]

http://www.harkavagrant.com/history/YoungDarwin.jpg

[Tristam]

I wondered if anyone could help me. Science has always interested me but i've never really gone any further than having an interest. Looking for somewhere on the web where i could learn some pretty basic science, any decent sites for this kind of stuff?

I would suggest starting with reading general popular science books, Bill Bryson's "A short history of nearly everything" is a fun way to get interested in science in general. Then you'll want to identify what specific fields interest you. If you find yourself drawn more to biology, Stephen Jay Gould or Dawkins have some very good books out there. If you are more the physics type, there are others here who can point you in the right direction. Textbooks, of course, are the best way to gain a deeper understanding of any science but I think you'll want to have a general idea of what you like before you invest time and money in studying something.

edit: I fail reading comp, for online resources scienceblogs.com is a fantastic reference. Discover blogs is a great site, a bit more professional. There are other blogging communities out there run by scientists but I really think those two are the best. Also, many profs post lectures online and you might be able to get to look at them. I used random lecture slides as study aids when I was an undergrad and needed other resources to help learn something.

[Eliseos]

Also, many profs post lectures online and you might be able to get to look at them.

These are going to be pretty good for giving you an idea of the classes, should you choose to pursue a science. Open Courseware from MIT is also awesome. You can look up videotaped lectures on the iTunes store and download an entire course for free. They have a pretty large variety of topics/courses also. Science, Engineering, Mathmatics, etc. I used them as supplemental material for the actual classes I was taking, and it helped to have a different perspective on things.

[Mizango]

I wondered if anyone could help me. Science has always interested me but i've never really gone any further than having an interest. Looking for somewhere on the web where i could learn some pretty basic science, any decent sites for this kind of stuff?

I always suggest this book for starters since it is technical, but is a very easy and delightfully fascinating read.

http://www.amazon.com/Origins-Fourte.../dp/0393327582

You're welcome.

[idklol]

Thanks for the replies. Should of been a bit more specific in my last post. What i enjoy most is space and anything to do with it. Always been fascinated by it, i just never had the desire to pursue it at school.

Truth be told i'm not exactly happy in my job and would really like a change. Been looking into my interests to see where i could go but i'm not sure of where to be looking career wise. Finding some good reading material would not only interest me but could be a career move if possible.

[Altariel]

Thanks for the replies. Should of been a bit more specific in my last post. What i enjoy most is space and anything to do with it. Always been fascinated by it, i just never had the desire to pursue it at school.

I meant to reply to this when you first asked and I was busy at the time. It is 30 years old but Carl Sagan's Cosmos is a very good read.

[Woozie]

Watch his videos too. You're not a true astronomy/space fan until you can do a Carl Sagan impersonation.

[Caiti]

Watch his videos too. You're not a true astronomy/space fan until you can do a Carl Sagan impersonation.

My thoughts exactly, Sath sparked my interest in astronomy by getting me to watch Cosmos.

[Waef]

So I'm having trouble figuring out this problem on the study guide for my Cal3 test. It's about line integrals, but we never covered how to solve them when the curve is a function of x and y rather than a line already parameterized by t, and the book doesn't seem to offer any sort of explanation. If anyone would be willing to help and explain how to do it that would be awesome. Problem:

Evaluate the line integral of f(x,y) along the curve C.

f(x,y) = x^2 + y^2
C: y = -2x-3
0 =< x =< 3

[Woozie]

Do you know how to parametrize the curve? Can you write x and y as a function of t? (Actually, if you look at it, you'll notice it's already parameterized, but in terms of x instead of t. If the x's and y's confuse you, write every x as t and every y as -2t-3).

Edit: And if you see anything like this on your test (where y is given in terms of x), you can always do the same thing. Just rewrite x as t and y(x) as y(t). If instead x is given in terms of y, then let y be t and x(y) be x(t)

If y is not given in terms of x, you can usually look at the expressions and figure out a way to write either y in terms of x or x in terms of y. But I'm assuming this wont happen on your test since your professor hasn't gone over parameterization yet.

[Waef]

Ah, that's really simple. Thank you though, it worked. :D

[Max™]

Anyone remember hearing about SN1979 C?

The supernova was very interesting as it fit certain models suggesting it should produce a black hole.

After decades of observation, it seems NASA has found evidence that there is a black hole feeding where SN1979 C was.

The 30-year-old object is a remnant of SN 1979C, a supernova in the galaxy M100 approximately 50 million light years from Earth. Data from Chandra, NASA's Swift satellite, the European Space Agency's XMM-Newton and the German ROSAT observatory revealed a bright source of X-rays that has remained steady during observation from 1995 to 2007. This suggests the object is a black hole being fed either by material falling into it from the supernova or a binary companion.

"If our interpretation is correct, this is the nearest example where the birth of a black hole has been observed," said Daniel Patnaude of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. who led the study.

The scientists think SN 1979C, first discovered by an amateur astronomer in 1979, formed when a star about 20 times more massive than the sun collapsed. Many new black holes in the distant universe previously have been detected in the form of gamma-ray bursts (GRBs).

However, SN 1979C is different because it is much closer and belongs to a class of supernovas unlikely to be associated with a GRB. Theory predicts most black holes in the universe should form when the core of a star collapses and a GRB is not produced.

"This may be the first time the common way of making a black hole has been observed," said co-author Abraham Loeb, also of the Harvard-Smithsonian Center for Astrophysics. "However, it is very difficult to detect this type of black hole birth because decades of X-ray observations are needed to make the case."

The idea of a black hole with an observed age of only about 30 years is consistent with recent theoretical work. In 2005, a theory was presented that the bright optical light of this supernova was powered by a jet from a black hole that was unable to penetrate the hydrogen envelope of the star to form a GRB. The results seen in the observations of SN 1979C fit this theory very well.

Although the evidence points to a newly formed black hole in SN 1979C, another intriguing possibility is that a young, rapidly spinning neutron star with a powerful wind of high energy particles could be responsible for the X-ray emission. This would make the object in SN 1979C the youngest and brightest example of such a "pulsar wind nebula" and the youngest known neutron star. The Crab pulsar, the best-known example of a bright pulsar wind nebula, is about 950 years old.

http://www.nasa.gov/home/hqnews/2010...9_CHANDRA.html