# Search results

1. ### Rod resting on a cylinder, find mu

My solution is on the files. The basic approac was the take moments about A. Then find the reaction at C. Use the reaction at C to find the friction between rod and cylinder. I take it to mean from the question that the friction between the rod and plane is equal to the friction between...
2. ### Index laws question

Homework Statement we are asked to compare the graphs of 0.5^x with 2^x using index laws Homework Equations I am not sure how the regular index laws apply The Attempt at a Solution All I can say at the moment is the the graph of 2^x is a reflection of 0.5^x in the Y axis. I don't know...
3. ### A question about ionic bonding

I am sure this question will have been asked elsewhere, so please forgive me if it has. My question involves the transfer of electrons in ionic bonding. We are told that a metal will give up its outer most electron because by doing so it will gain the electron configuration of a noble gas...
4. ### Statistics Permutations and Combinations

Homework Statement The back row of a cinema has 12 seats, all of which are empty. A group of 8 people including Mary and Francis, sit in this row. Find the number of ways they can sit in these 12 seats if a) There are no restrictions b) Mary and France's do not sit in seats which are next to...
5. ### Probability Question -- probability that no member will receive more than one of the medals

Homework Statement There are 15 members of a maths club. There are 4 different medals to be randomly given to the members of the club. What is the probability that no member will receive more than one of the medals. Homework Equations Try to find the number of combinations where no member...
6. ### Estimates of Percentage Errors in Cosomology

Homework Statement Suppose that there is some hypothetical cosmological parameter called C, and suppose that the real value of C is C(real) = 1. Also suppose that everyone in the universe is able to measure C. Our value of C is C(ours) = 1.15 +/- .15 at confidence level 95 % Make an...
7. ### I Variations in the local value of Hubble constant

I have been reading in various sources that the directly measured value of Ho is high than the global value inferred by precise measurements of the CMB. The local value seems to be around 73 km/s/Mpc where as the global value inferred, assuming a flat Lamda CDM model is about 67 km/s/Mpc...
8. ### I Cosmological Paramaters

Cosmological paramaters in the cold dark matter Lamda concordance model. I have been trying to find the most uptodate values for Ho , Ohmega m and Ohmega Lamgad I was wondering if anyone knows where I might find the most reliably, widely accepted values for these values that are currently...
9. ### I Lyman Break drop-out technique

Hi I have a question which is about the lyman break drop-out technique for identifying high red shift galaxies. I am basically trying to understand how the technique works. My current understanding is that the drop-out technique makes use of the fact that very high red shift galaxies...
10. ### I A basic question about units

Hi This is a really basic question about units. I have been reading a paper about supernova rates and the units are given as: SN rate per yr MPC^3. h_70^3 (10^ -4) Could someone tell me what the h subscript 70 to the power of 3 means? Thanks
11. ### B Dark Matter Invention

Ok - I have qualified this question with the word stupid so please go gently with me. Imagine you are talking to a rather stupid child when answering please. My question is about dark matter. Every time I do a search for "proof of dark matter", I get the same answer, which is that using the...
12. ### I How did scientists figure out the expansion is accelerating?

I understand that the acceleration of the universe's expansion was discovered by looking at very far away Type 1a supernovae. My question is how was the data used exactly to calculate the hubble constant in the past and then compare it with today's value? Did they simple plot the distances...
13. ### I Euclidean differential number counts of supernovae

Hi I am working on an assignment which is has asked us to derive an expression for a differential number count of supernovae in a euclidean flat non-expanding space. I am bit perplexed by this question and am wondering whether it is a trick question. We are allowed to do research to find an...
14. ### B Carbon dioxide and global warming

Hi I have a simple question about global warming. The percentage of carbon dioxide in the earth atmosphere is about 0.04% . My question is how is such a small percentage of this gas able to have such a powerful affect on global warming? Thanks
15. ### B Resonance of air between two loudspeakers

A student I am working with showed me a problem they have been working on. In the first part of the question there are two loudspeaker facing each other. The student has been told that they can treat this situation as an closed tube. The frequency of the note is such that its wavelength is...
16. ### I What is this equation?

Could someone tell me what the attached equation is called, and just and brief description of what it states. I don't recognise it and it's hard to know where one goes to just look up an equation. Thanks
17. ### I Cosmic Microwave Background Radiation Anisotropies

Hi This question may have already been answered elsewhere. If so please accept my apologies in advance. I am confused! The textbok(s) I am reading describe a whole bunch of different causes for there being temperature fluctuations in the CBM, so I am confused about which one(s) of these...
18. ### Nuclear decay of a small number of atoms calculation

I came across a rather dubious question that a teacher had put in a power point. It said something like,"Given a sample of 100 atoms of isotope x, after one half life of the said isotope, how many atoms of the original isotope will be left?" My answer was that it was a trick question because...
19. ### What are labels?

In QM, position is an operator, while in QFT, it is a label. Could someone help me understand what is meant by the term "label" - just in the simplest terms possible? Cheers
20. ### A dodgy derivation?

I have attached a photo of a page from a book that I am studying. The author is showing a derivation of E(ave) = kT However, I can't follow this derivation. He says that you need to use integration by substitution and I am OK with that. But when he actually does the substitution it looks...
21. ### Why does the Sun behave like a black body?

My question is, given that the Sun is composed almost entirely of just hydrogen and helium, why is the spectrum from it continuous and not an emission line spectrum? This applies to other stars as well. There are other elements of course present in the Sun, but they are present in very small...
22. ### What is the opposite of a field theory?

Hi I understand that Maxwell's equations, and General Relativity are both field theories. I am trying to understand what the opposite of a field theory is, or what is not a field theory. For example, am I correct in believing that Newton's Laws are not a field theory? Is that is true then...
23. ### Slightly confusing equation

I am reading a book about quantum mechanics and the author is trying to show how the uncertainty principle can be obtained from the Schwarz inequality. Without going into the whole thing there is one point I get stuck following what he is doing. We have an operator called DA. We start with...
24. ### Derivation of standard model lagrangian

What's the simplest, most direct way to derive the lagrangian of the SM? I saw earlier today: L[S M] = L[Dirac] + L[mass] + L[Gauge] + L[Gauge/psi] That seems like a good starting point. I like it because it says the SM Lagrangian is simply the sum of four lagrangians. The next step...
25. ### Simple explanation of group generators

Hi I am trying to get a simple grasp of the concept of a group generator and group representations. As ever wherever I look, I get very mathematical speak definitions such as: "A representation is a mapping that takes elements g in G into linear operators F that preserve the composition...
26. ### Simple differential equation

Homework Statement Given that a particle has an initial velocity v0 and then undergoes an acceleration a = - bv. , where b is a constant, obtain an expression for v = v(t) and x = x (t) [/B] Homework Equations Not sure The Attempt at a Solution If I integrate a = - b v I think I get v...
27. ### A good place to find out the meaning of basic math terms

Hi I have trying to study independently some fairly high level physics but I keep getting bogged down with simple (I imagine for other people) mathematics terms: Here is a partial list of terms I don't get: Operator Argument Mapping Comutator There are probably many others. My question is...
28. ### How do we see the color violet?

If our eyes have only three types of color detector: one for red, blue and green, how do we perceive the color violet when we are looking at a spectrum? If I am looking at the iphone or a TV or other device which effectively only emits combinations of pure red, blue and green light, I can see a...
29. ### Density perturbations made simple

Hi I know that this question has been answered in other threads, but those threads seemed to be closed now, and what I am looking for is a real "for beginner - real dummies" type answer. I understand that there are ripples in the background microwave radiation. The text book (OU) I am...
30. ### Cosmic rest frame, comoving distance, and proper distance

Hi I am trying to understand more about these terms. I am currently studying a course about relativity and cosmology, but I am finding the textbook (Open University) difficult to follow. Can anyone help me untangle and make some simple sense of these different terms? Thanks (I accidentally...