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Error Propagation

  1. Nov 15, 2008 #1
    1. The problem statement, all variables and given/known data

    Given that a puck's velocity is speed [tex]v[/tex] at an angle [tex]\theta[/tex] (measured in radians) with the x-axis, we know that the puck's x-velocity is [tex]v\cos(\theta)[/tex]. Given the error in [tex]v[/tex] is [tex]\sigma_v[/tex] and the error in [tex]\theta[/tex] is [tex]\sigma_\theta[/tex], what is the resulting error in the puck's x-velocity?


    3. The attempt at a solution

    Solving for partials, we get:

    [tex]\sigma_{v_{x}}=\sqrt{\left(\cos(\theta)\sigma_{v}\right)^2+\left(-v\sin(\theta)\sigma_{\theta}\right)^2}[/tex].

    Or, using proportionalities of errors, we find:

    [tex]\sigma_{v_{x}}=\sqrt{\left(\frac{\sigma_{v}}{v}\right)^2{v_{x}}^2+\left(\frac{\sigma_{\theta}}{\theta}\right)^2{v_{x}}^2}[/tex].

    These yield two different values... Which is a real propagation?
     
  2. jcsd
  3. Nov 15, 2008 #2

    LowlyPion

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    Homework Helper

    This link touches on dealing with error propagation for angles.
    http://instructor.physics.lsa.umich.edu/ip-labs/tutorials/errors/prop.html

    Since you are interested in the product of two measured values that would suggest that your second method would be the final step.

    But arriving at the fractional uncertainty of the Trig function suggests finding the absolute uncertainty in the function first.

    By the Rule 4 at the link I cited above you might model that as σf = dF(θ)/dθ = σθSinθ

    From that calculate the relative uncertainty as σθSinθ/Cosθ = σθTanθ ?

    By my method I think that would make it

    σvx = ((σv/v)2 + (σθTanθ)2)1/2
     
  4. Nov 15, 2008 #3
    Thank you, Lowly, but I guess I'm still confused. All year we've used the RSS of the partials to show the error in the unknown parameter but, recently, my TA introduced the idea of proportionality of squares. That is, [tex](dC/C)^2 = (dA/A)^2+(dB/B)^2[/tex]. Which is right?
     
  5. Nov 15, 2008 #4

    LowlyPion

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    By my method I think that would make it

    σvx = ((σv/v)2 + (σθTanθ)2)1/2
     
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