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  1. QuarkDecay

    I Horizon Problem

    It says that since there is homogeneity in the Universe's temperature, all these points must have come from one source (or a source close to each other?) at a certain time. Then it also calculates the number of these sources and it's ~105. But isn't that very dense mass right before the Big...
  2. R

    Three masses, a string and pulley and a table -- solve for the acceleration

    Let T be the tension in the string, a be the acceleration of mass 2m, 2a be the acceleration of mass m T = (m) (2a) ---eq(1) The mass 3m will come down with acceleration a’ = (a+2a)/2 = 3a/2 3mg - 2T = 3m . 3a/2 from equation 1 3mg - 2(2ma) = 3m . 3a/2 thus a = 6/17g thus acceleration of 3m...
  3. S

    Which answer is correct? Grocery Bag Net Force Question

    Today, my class had a dispute between who was right. Two answer keys for this problem was provided, so nobody knows the exact answer to this problem. Half of my class said 100 N while the others said 298 N or something. We won't talk about how I wasn't listening to that because I was trying to...
  4. H

    Force exerted on the horizontal bottom of the container

    Homework Statement In an empty container, having the shape of a Rectangular Parallelepiped, with width l=0.5m and MN=H=0.4m, and NP=L=0.75M. we poureda mass of mercury =2448kg. T0,T1,T2 have the same section which is 0.2 m^2. we give g=10N/kg, and density of mercury u=13.6 g/(cm^3)...
  5. tomalku

    Equation of scaler potential problem

    Homework Statement Homework Equations Equations of Lienard Wiechert Potential The Attempt at a Solution I had used the equation of scaler potential. I don't understand how to use angle 60 degree is this equation. suggestion needed
  6. SemM

    Suggestion Article on the Quantum Marginal problem

    Hi, would someone be willing to write a pedagogic article on the Quantum Marginal Problem? Thanks!
  7. G

    Can i solve this with the Theorem of thevenin?

    Homework Statement 1. Homework Statement [/B] Homework Equations [/B] Use thevenin method to find the intensity between A and B The Attempt at a Solution Well I tried a easier method but I just cant find the same answer[/B]
  8. MarcusAu314

    PIC16F877A I2C communication problem

    Good evening people, I was planing to make a programmable clock using a PIC16F877A microcontroller, a LM016 LCD display and a DS3232 RTC module. The DS3231 RTC module communicates with the PIC via I2C bus. At first I built some subroutines for controlling I2C communication: the commands have...
  9. MarcusAu314

    PIC Microcontrollers: problem with LCD module (see below)

    Good morning people. Recently I started programming in assembly language my PIC16F877A. Success. Now I wanted to work with a LCD display (more specifically a 16x2 LM016 display). At first I did not know the commands until I saw a datasheet and this page...
  10. A

    How To lock My gear?

    SO, here is the case guys. I have a mechanism to turn plate by a motor. Both motor and plate was connected through 2 spur gears. But here is the problem, when wind strike the plate, the plate start to rotate even without the power of the motor. What is the best way so i can prevent the plate...
  11. Anshul23

    Highschool graduate dealing with a triple integral?

    I recently came across a problem in Irodov which dealt with the gravitational field strength of a sphere. Took some time to get my head around it and figure how to frame a triple integral, but it felt good at the end. Am I going to start seeing triple integrals in the freshman year tho? If so...
  12. K

    I Problem Hubble parameter

    This is the time derivative to calculate the speed which a galaxy moves away from another galaxy. I don't understand how they get from da/dt (xi − x1) to (∙a)/a a(t). (xi − x1). Could anyone explain this? vi(t) = d/dt (ri(t) − r1(t)) = d/dt a(t)(xi − x1) = da/dt (xi − x1)...
  13. D

    Small and powerful enough battery?

    Hi all, I work on something.. its alarm tag. I found buzzer which i want to use in simple electric circuit. Components are the buzzer, and a battery. The tag must work minimum 5 min. The problem is.. I can not find battery small and powerful enough at the same time. Battery diagonal must be 3...
  14. doktorwho

    Two combinatorics problems

    Homework Statement Using the common logic and known combinatorics properties solve the following problems: a) If you have 6 letters and 6 envelopes, each having it's corresponding envelope in how many ways can you place every letter in the wrong envelope? (1 letter goes into 1 envelope) b)You...
  15. W

    Studying Bad Problem-Solving Habits

    Hi all, I am an undergraduate Physics student. I have recently realised that I am extremely bad at solving "new" problems related to the topics I am working on. Trying to do any question that deviates slightly from the norm will cause me to be stuck on that problem for hours, usually to no...
  16. B

    Semi-empirical mass formula: most stable isobar for odd A

    Homework Statement Using the Semi-empirical Mass Formula show that for fixed odd atomic mass number, A, the most stable isobar has a neutron to proton ratio given by N/Z = 1 + aCA2/3/(2aA) Homework Equations B(A,Z) = aV A − aS A2/3 − aCZ2A-1/3 − aA(Z − N)2/A + pairing term The Attempt at a...
  17. peroAlex

    Current density of specific configuration

    Homework Statement Two hollow metal spheres (left one has radius of 0.7 meters and right one has radius of 0.4 meters) are half-submerged in the sea water with specific conductance of γ = 4 S/m. Distance between their centers is much greater than their radii dimensions and equals 60 meters...
  18. T

    I Ramanujan problem

    Hi everyone. This is my proof (?)of ramanujan's problem 525: (link to problem) [![enter image description here][1]][1] $$ \sqrt{A^{1/3}-B^{1/3}}=\frac{(A*B/10)^{1/3}+(A \times B)^{1/3}-(A^2)^{1/3}}{3} \Leftrightarrow \\ 9...
  19. doktorwho

    Engineering Help understanding the solution to a circuit problem

    Homework Statement When the switch is open, the voltage is ##U_{ab}##. When the switch is closed the calculated max current is ##I_{max}##. Calculate the resistance of ##R_p## when its power is max and the current of ##I_g## ##R1=100,R3=200,R4=400##,##0<Rp<100## and...
  20. TheBlackAdder

    B Peeking at the answer of an unsolved problem.

    Should peeking at the answer of an unsolved problem be avoided at any cost? I'm self studying at the moment and have a document with the solutions. It doesn't happen frequently, but I have had to peek at answers a couple of times. I really hate doing it though. Thoughts?
  21. James Kent

    Device for rotating a cord

    This is a thought experiment: My PC is mounted on my ceiling. I want to run a USB peripheral through a cylindrical retraction device (like a dog leash). The device is going to mount to the ceiling also and rotate about it's vertical axis. This will allow me to pull down the mouse from the...
  22. anon11

    Ball Rolling Without Slipping On A Turntable

    Can someone please help me out with this tough problem? 1. Homework Statement A ball rolls on a turntable without slipping describe the balls motion in terms of (x,y) for a function of time. (The turntable spins at a constant rate) (This all the information that the professor gave us.) The...
  23. A

    Need help with Mechanic Energy math problem.

    Homework Statement "A little girl is sliding down a snowy hill on her sledge and out on a horizontall flat area. The girl has a physic intrested mom which wants to know what the maxium speed the sledge can reach. She measures that the hill is 3,0m high and 6,0m long. The sledge slides for 12.5m...
  24. Denny Lai

    Combining 2 AC power suppy

    As per the diagram, how do i change the wiring of the equipment on the outside to achieve my desired setup?
  25. MarcusAu314

    Control theory: block diagram, problem (detailed below)

    I´m taking a course on control engineering and I have a test next Tuesday so I need to study the basics which are: Laplace transform, simplification of block diagrams, and analysis of transient and steady state responses. Right now I am dealing with the second one. I know the basic rules of...
  26. MarcusAu314

    BJT transistor analysis: common emitter confusion

    Recently I started studying semiconductors and analogue electronics. First I studied the diode and its I-V characteristics and analysis in DC circuits, as well as in AC circuits. Now I started with BJT transistors. I was explained that transistors act as electronic switches. My problem is not...
  27. Anonymous Vegetable

    B Implications of e^i*pi = -1

    Before I start, there are only really two pieces of information this concerns and that is the idea that 1x = 1 and that ei*π = -1 So it would follow that (ei*π)i = -1i And so that would mean that i2i = e-π which doesn't seem to be right at all. Where is the issue here as there must be one but I...
  28. Mastermind01

    Studying When is a good time to give up on a problem?

    Hello all, Apologies if this has already been asked. While studying, I often encounter hard problems, problems that I can't solve after repeated tries. When is a good time to give up and succumb to seeing the solution or asking someone? Ideally, I think the answer is never. But I have a...
  29. RoboNerd

    Finding an equation of the tangent plane -- need help with steps

    Hi everyone. I am told to find the equation of the tangent plane to the surface x^2 + 2xy^2 -3z^3 = 6. I do not know how to approach this problem, and I was wondering if anyone would be kind enough to help. I know that for example if I had an equation z = x^2 + y^2, with a point P(x0,y0) and...