What is Variable: Definition and 1000 Discussions

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. 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...
  2. 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?
  3. A

    Expected value of a function of a random variable

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  4. T

    Atwood machine with variable mass

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  5. EEristavi

    B Random Variable - Mean and Variance

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  6. Robin04

    First-order homogeneous recurrence relation with variable coefficient

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  7. F

    Curve and admissible change of variable

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  8. S

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  9. J

    Variable mass beans falling on a platform

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  10. F

    Dimensional analysis-how one variable scales w/ another

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  11. astroman707

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

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  12. giveortake

    Calculating the work done from an equation for variable force

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  13. m4r35n357

    GCC warning: function may return address of local variable

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  14. FallenApple

    A Creating a confounding variable

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  15. 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?
  16. 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...
  17. jk22

    B Is Bell's parameter really a hidden variable ?

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  18. A

    B Is string theory a deterministic hidden variable theory?

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  19. jxj

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  20. navneet9431

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  21. 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. $\begin{bmatrix} 1 &-1 &|1 \\ 3 &a &|3 \end{bmatrix}$...
  22. 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...
  23. S

    B An elementary confusion on discrete or continuous variable

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  24. Scrope

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  25. sweet springs

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  26. V

    Calculus Best textbook to truly understand single variable calculus?

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  27. Peter Alexander

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  28. entropy1

    B Random variable reflecting its probability

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  29. P

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  30. 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?
  31. 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...
  32. 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...
  33. 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...
  34. 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...
  35. 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...
  36. 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...
  37. 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}...
  38. 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...
  39. M

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

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  40. 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) #...
  41. Math Amateur

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

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  42. 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...
  43. 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]
  44. 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...
  45. 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...
  46. 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...
  47. M

    MATLAB Numerical Integration with variable limits MATLAB

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  48. opus

    Surface area of a cylinder- finding unknown variable

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  49. M

    MHB Proving Skewness of a Random Variable $X$

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  50. D

    How Does the Brightness and Temperature of l Carinae Change with Its Pulsations?

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