Variable Definition and 1000 Threads

A variable star is a star whose brightness as seen from Earth (its apparent magnitude) fluctuates.
This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as either:

Intrinsic variables, whose luminosity actually changes; for example, because the star periodically swells and shrinks.
Extrinsic variables, whose apparent changes in brightness are due to changes in the amount of their light that can reach Earth; for example, because the star has an orbiting companion that sometimes eclipses it.Many, possibly most, stars have at least some variation in luminosity: the energy output of our Sun, for example, varies by about 0.1% over an 11-year solar cycle.

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  1. A

    Variable induced surface charge density on a sphere

    Homework Statement Homework Equations While solving this problem at r >>a ,the corresponding potential due to the dipole is kpcosθ/r2(potential due a dipole) where k is the electrostatic constt. ...(1) If σ(θ) is the surface charge density induced due to external electric field. then the...
  2. R

    I Using PCA for variable reduction

    In the textbook “Principal Component Analysis” Jolliffe (§9.2) suggests the following method for variable reduction: “When the variables fall into well-defined clusters, there will be one high-variance PC and, except in the case of 'single-variable' clusters, one or more low-variance PCs...
  3. J

    Vector Plasma vs. Variable Radioactive Decay

    Could there be a connection between Robert Zimmermann's work (McMaster Univ. Toronto) on Vector Plasma, and Jenkins and Fischbach's (Perdue Univ.) work on variations in the rate of radioactive decay for elements on Earth in relation to solar activity? Only looking for a confirmation that their...
  4. M

    Mathematica Plotting 0.3 on ListPlot with Variable Domain

    Hi PF! I type the following into Mathematica: domain = -10; ListPlot[{0.3}, PlotMarkers -> {{"\[LongDash]", Large}}, PlotRange -> {{0, 5.5}, {-10, 200}}] And notice we see the plot 0.3. But, now let domain = 0. Notice now we can't see 0.3. Does anyone know how to fix this?
  5. A

    Expected value of a function of a random variable

    Homework Statement Let X be a random variable. It is not specified if it is continuous or discrete. Let g(x) alway positive and strictly increasing. Deduce this inequality: $$P(X\geqslant x) \leqslant \frac{Eg(X)}{g(x)} \: $$ where x is real. Homework Equations I know that if X is discrete...
  6. T

    Atwood machine with variable mass

    Homework Statement We have an Atwood machine like the picture below. one side (left) is a bucket full of water which has a hole on the bottom and the water is flowing with rate ##dm/dt = \alpha = const##. The initial mass of bucket with the water is ##m_0##. On the other side (right) we have a...
  7. EEristavi

    B Random Variable - Mean and Variance

    Problem: We play roulette in a casino. We watch 100 rounds that result in a number between 1 and 36. and count the number of rounds for which the result is odd. assuming that the roulette is fair, calculate the mean and deviation Solution: I understand that the probability - Pr = 0.5. and...
  8. Robin04

    First-order homogeneous recurrence relation with variable coefficient

    Homework Statement I need to find the explicit formula for the following recursive sequence: ##v_n=\frac{2}{1+q^n}v_{n-1}## where ##0<q<1## is a constant Homework Equations I found the following method to solve it...
  9. F

    Curve and admissible change of variable

    Homework Statement If I have the two curves ##\phi (t) = ( \cos t , \sin t ) ## with ## t \in [0, 2\pi]## ##\psi(s) = ( \sin 2s , \cos 2s ) ## with ## s \in [\frac{\pi}{4} , \frac{5 \pi}{4} ] ## My textbook says that they are equivalent because ##\psi(s) = \phi \circ g^{-1}(s) ## where ##...
  10. S

    Java Updating an index variable in Java

    For the life of me, I can't figure out why my program won't update the index variable. I've tried setting the index to 0, and then to -1, and to the size of the arrayList - 1. I'm trying to use the index to get the previous and next flashCard in the arrayList to show up. I have been working on...
  11. J

    Variable mass beans falling on a platform

    Homework Statement A spigot pours beans onto a scale platform. At a time t = 0.0 sec, the spigot is opened and beans begins to pour out (with initial velocity 0) at a rate of 1.00 kg/sec onto the platform from a height of 10.0 m above. (a) At t = 10.0 sec, what is the weight of beans on the...
  12. F

    Dimensional analysis-how one variable scales w/ another

    Homework Statement In PSet 3, Prob. 1(e), you determined the thickness h of a thin film of fluid (of viscosity µ and density ρ) flowing down an incline of angle θ, driven by gravity (acceleration g), via a supplied upstream flow rate Q per unit width (i.e., [Q] = (length)2/time). (a) Using...
  13. astroman707

    Python What's wrong with my if-else statements in Python 3?

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  14. giveortake

    Calculating the work done from an equation for variable force

    Homework Statement The force on a particle is directed along an x axis and given by F = F0(x/x0 - 1) where x is in meters and F is in Newtons. If F0 = 1.5 N and x0 = 4.9 m, find the work done by the force in moving the particle from x = 0 to x = 2x0 m. Homework Equations F = force, w = work, x...
  15. m4r35n357

    GCC warning: function may return address of local variable

    Yes I've already looked on SO! None of the answers contain the word "may". My story is that I am passing in references to global variables, modifying them, and returning them in array literals. I suppose it must complaining about the array literals (c99) rather than the variables themselves...
  16. FallenApple

    A Creating a confounding variable

    So I have Y, the response and X1 and X2. I generate Y and X1 from a multivariate normal distribution. Then I manually set X2 to be nearly same as X1( the same except for the fact that I change up a few entries to make X2 distinct from X1). I ran three separate linear regressions. lm(Y~X1)...
  17. R

    MHB Exponents and Variable Equalities

    I am not sure how to figure this out. Basically, I know XN + XN = 2XN and that 2N + 2N = 2N+1. So, since 2 * 2 = 4 why doesn't 2N+1 = (2 * 2)N = 4N?
  18. D

    Derivative of expanded function wrt expanded variable?

    Homework Statement If I have the following expansion f(r,t) \approx g(r) + \varepsilon \delta g(r,t) + O(\varepsilon^2) This means for other function U(f(r,t)) U(f(r,t)) = U( g(r) + \varepsilon \delta g(r,t)) \approx U(g) + \varepsilon \delta g \dfrac{dU}{dg} + O(\varepsilon^2) Then up to...
  19. jk22

    B Is Bell's parameter really a hidden variable ?

    In the Ansatz : $$C(a,b)=\int A(a,s)B(b,s)ds$$, is $$s$$ not simply the mute summation index ? In this sense it is not hidden to the experiment. Then what about an "dummy" hidden variable that is not integrated over : $$C(a,b)(\phi)=\int A(a,s,\phi)B(b,s,\phi)ds$$ $$\phi$$ were tuned one and...
  20. A

    B Is string theory a deterministic hidden variable theory?

    There are several interpretations of QM which differ from being deterministic or non deterministic, with or without hidden variables and local or non-local. As I understand it ST poses the existence of vibrating strings, that is, physical objects with definite properties, moving along a world...
  21. jxj

    Isolating Variable in Equation for conservation of momentum

    Homework Statement So the problem is trying to isolate mA in the equation for momentum (only focusing on top formula, not bottom hehe) basically by solving the equation I assume. My teacher said because the vA and vB on the right were prime they could not be combined so I'm having trouble...
  22. navneet9431

    Calculation of work done by a variable force

    Homework Statement See the explanation of work done by variable forces given in my textbook. For calculation of work done by variable forces the author analyses only a small fraction of displacement "ds". And then he makes **assumption** that the force is acting on the particle at an angle...
  23. T

    MHB Matrix solutions with variable coefficient

    Hey guys, I've got this question, that I think I have figured out, but I'm not completely sure. Basically, I've got the following matrix to put into RREF. Not really a problem, but I'm not sure if I'm handling the variable incorrectly. $\begin{bmatrix} 1 &-1 &|1 \\ 3 &a &|3 \end{bmatrix}$...
  24. U

    Applying W=F*s for variable force

    Homework Statement A car of mass 2000kg moves along a horizontal road against a constant resistance of manitude (P)N. The total work done by the engine in increasing its speed from 4ms^-1 to 5.5ms^-1 while it moves a distance of 60m is 30000J. Find P. Homework Equations ΔEk+WP=WE The Attempt...
  25. S

    B An elementary confusion on discrete or continuous variable

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  26. Scrope

    Solve Multi Var Limit: Help Appreciated!

    Homework Statement https://gyazo.com/268bef206850bfbf30fb0cca3f783599 <----- The question Homework EquationsThe Attempt at a Solution Had this on a test today, honestly not sure how to evaluate. I know you can pass the limit to the inside of arctan but I can't see how the inside goes to...
  27. sweet springs

    Exchanging roles of variable and thermal potential

    In thermal physics entropy S is regarded as a system variable in 1st theorem of thermal dynamics, i.e. dE(S,V)=TdS-pdV=T(S,V)dS-p(S,V)dV Entropy is regarded also as thermal potential,i.e. dS(E,V)=\frac{1}{T(E,V)}dE + \frac{p(E,V)}{T(E,V)}dV S,V,p,T are usually regarded as variables. My...
  28. V

    Calculus Best textbook to truly understand single variable calculus?

    I am currently self learning high school level AP Calculus BC over the summer. From a mathematical perspective, I have heard that high school level calculus is just pure and shallow computational work. For this reason, I seek a deeper understanding in calculus. So far, the BC curriculum has been...
  29. Peter Alexander

    Solving Second Order Partial Derivative By Changing Variable

    1. The problem statement, all variables, and given/known data Given is a second order partial differential equation $$u_{xx} + 2u_{xy} + u_{yy}=0$$ which should be solved with change of variables, namely ##t = x## and ##z = x-y##. Homework Equations Chain rule $$\frac{dz}{dx} = \frac{dz}{dy}...
  30. entropy1

    B Random variable reflecting its probability

    If we have a series of, say, twenty coin tosses, then each discernable specific series of outcomes has equal probability to occur. However, there is only one discernable specific series consisting of twenty 1's, while there are many more discernable series consisting of ten 1's and ten 0's. So...
  31. P

    Elastic collision problem (μk) is the target variable

    Homework Statement Two certain small objects A and B, each have mass m. A is suspended using a lightweight, inelastic string, and B is placed on a rough horizontal floor in a position where it is in contact with the vertically suspended A. Next, A is raised to height h above its original...
  32. granzer

    Why variable geometry nozzles are not used on rocket engines

    Why is it that variable geometry nozzles, like those found on jet engine(iris nozzles), are not used as rocket nozzle to provide better altitude compensation?
  33. opus

    Isolating the Y variable in a Trigonometric Equation

    Homework Statement ##4y=cos\left(4πx+\frac{3}{2}\right)## Homework EquationsThe Attempt at a Solution In dividing both sides by 4, I got: ##y=\frac{1}{4}cos\left(πx+\frac{3}{8}\right)## But I am told this is incorrect. Not sure if dividing everything by 4 here is an allowable technique, or if...
  34. J

    A Photon direction as hidden variable in a double slit expt?

    I was wondering if it would be feasible to set up something like the following experiment to validate the pilot wave theory. The idea here is to set up a double slit experiment, but control the initial conditions of photons emitted from a source (i.e., their directions) such that one can...
  35. evinda

    MHB Change variable to solve equation

    Hello! (Wave) I want to solve the equation $xu_t+uu_x=0$ with $u(x,0)=x$. There is a hint to change variables $x \mapsto x^2$. I have tried the following. Let $x=y^2$. Then $u_x=\frac{du}{dy} \frac{dy}{dx}+\frac{du}{dt} \frac{dt}{dx}=u_y \frac{1}{\frac{dx}{dy}}=\frac{1}{2y} u_y$. Thus...
  36. A. Neumaier

    I Relativistic hidden variable quantum mechanics?

    I just came across the following paper: Gisin, N. (2011). Impossibility of covariant deterministic nonlocal hidden-variable extensions of quantum theory. Physical Review A, 83(2), 020102. proving that, under sensible hypotheses, nonlocal hidden variable theories for relativistic quantum...
  37. opus

    B Doing proofs: Setting an expression as a variable

    In a book I'm reading, I'm told to prove that: If m and n are odd, then (m)(n) is odd. The proof goes as such: Let m=(2a+1) and n=(2b+1) Then, mn= (2a+1)(2b+1) = 4ab+2a+2b+1 = 2(2ab+a+b)+1 = 2t+1 ; where t= 2ab+a+b Two questions: When we take an expression, and assign it to a single...
  38. cosmik debris

    Automotive How is variable engine braking implemented in a vehicle ECU?

    Hi All, The ECU on my motor bike allows you to select different amounts of engine braking in it's various riding modes. I imagine that the ECU just opens up the throttle bodies a little so that at closed throttle the engine still delivers a bit of torque. Doing this will increase the idle speed...
  39. R

    How to know which variable comes first in the Jacobian?

    Homework Statement Find the Jacobian of the transformation: x = e^{-r}sinθ , y = e^rcosθ Homework EquationsThe Attempt at a Solution formula for Jacobian is absolute value of the determinant \begin{vmatrix} \frac {∂x}{∂u} & \frac {∂x}{∂v}\\ \frac {∂y}{∂u} & \frac {∂y}{∂v}\\ \end{vmatrix}...
  40. F

    Substitution in a differential equation, independent variable

    Homework Statement $$y'=-\frac{1}{10}y+(cos t)y^2$$ when doing substitute for ##z=\frac{1}{y}## I understand this is ##z(t)=\frac{1}{y(t)}## I know t is independent variable and y is dependent variable but I want to know what is z role here, is it change the dependent variable? when...
  41. M

    Hello, how can I replace a variable capacitor with a varicap

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  42. M

    Exploring Reyleigh-Taylor Instability with Variable Density and Viscosity

    Homework Statement import time import numpy as np from Output import * from PoissonSolver import * tmps1=time.clock() # Physical parameters Re = 10. # Numerical parameters CFL = 0.9 dt = 0.001/4 t = 0.0 tmax = 1.5 # Make mesh from MeshGeneration import * mesh = Mesh(2.0,1.0,50,25) #...
  43. Math Amateur

    MHB Differentiating Vector-Valued Function: Junghenn Prop 9.1.2 - Peter seeks help

    Derivative of a Vector-Valued Function of a Real Variable - Junghenn Propn 9.1.2 ... I am reading Hugo D. Junghenn's book: "A Course in Real Analysis" ... I am currently focused on Chapter 9: "Differentiation on \mathbb{R}^n" I need some help with the proof of Proposition 9.1.2 ...
  44. Math Amateur

    I Derivative of a Vector-Valued Function of a Real Variable ...

    I am reading Hugo D. Junghenn's book: "A Course in Real Analysis" ... I am currently focused on Chapter 9: "Differentiation on \mathbb{R}^n" I need some help with the proof of Proposition 9.1.2 ... Proposition 9.1.2 and the preceding relevant Definition 9.1.1 read as follows: In the above...
  45. chwala

    Solving four variable simultaneous equations

    Homework Statement given ##mx + ny =7## ##mx^2 + ny^2=49## ##mx^3+ny^3=133## ##mx^4+ny^4=406## find ##2014(x+y-xy)-100(m+n)## Homework EquationsThe Attempt at a Solution ##(mx+ny)(mx-ny)=7(mx-ny)## ##m^2x^2-n^2y^2=7(mx-ny)## ##mx^2+ny^2=49##[/B]
  46. CopyOfA

    A Linear regression with discrete independent variable

    Hey, I have a problem where I have a discrete independent variable (integers spanning 1 through 27) and a continuous dependent variable (50 data points for each independent variable). I am wondering about the best method of regression here. Should I just fit to the mean or median? Is there a way...
  47. M

    Need to find an old variable capacitor

    hello i buy allmost all thing needed, for buil you own radio circuits 2013, but in electronics shop he told me , there are old, you can find it only in old radio he can't help, those components don't sell anymore, maybe somebody can help me how to find online those component VC1: two-gang...
  48. S

    I Sanity Check: Four-Force & Variable Mass

    In classical mechanics, there's some debate about whether to define force as ##\mathbf{f} = m\mathbf{a}## or as ##\mathbf{f} = \dot {\mathbf{p}} = d (m\mathbf{v})/dt##. The former is much more popular I think, and it has the virtue of always having a Galilean-invariant magnitude ##f##, whereas...
  49. M

    MATLAB Numerical Integration with variable limits MATLAB

    Hi PF! Suppose I have two functions ##f(x),\,g(y)## that are numerically defined as vectors (i.e. ##g(y) = [0,1,4,9,16]:y = [0,1,2,3,4]## and say ##f(x) = [0,1,8,27,64]:x = [0,1,2,3,4]##) and am trying to compute $$\int_0^1 f(x) \int_x^1 g(y)\, dydx.$$ How would I do this in MATLAB? I could be...
  50. opus

    Surface area of a cylinder- finding unknown variable

    Homework Statement The surface area, A, of a cylinder with height, h, and radius, r, is given by the equation ##A=2πrh+2πr^2##. A company makes soup cans by using 32π square inches of aluminum sheet for each can. If the height of the can is 6 inches, find the radius of the can. Homework...
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