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Homework Help: Calculating uncertainty

  1. Jan 10, 2014 #1
    1. The problem statement, all variables and given/known data

    An object of mass m = 2.3±0.1 kg is moving at a speed of v = 1.25±0.03 m/s. Calculate the kinetic energy (K = 1 mv2) of the object. What is the uncertainty
    in K?

    2. Relevant equations


    3. The attempt at a solution
    I have figured out that the kinetic energy is 1.8 J, but how do I figure out the level of uncertainty for this question? We do not use derivatives yet.
    Can I take the equation for uncertainty of a power and uncertainy of a constant and add them together? :
    change in z= k change in x
    =1/2 * 0.1kg

    change in z= nx ^n-1 * change in x
    =2 * 1.25 ^2-1 * 0.03

    then add them together to give an uncertainty of + or - 0.125?
  2. jcsd
  3. Jan 10, 2014 #2
    When multiplying/dividing uncertainties, you just add the ratios in quadrature. But in my opinion it is easiest to just do
    σ[itex]_{k}[/itex][itex]^{2}[/itex] = (∂[itex]_{k}[/itex]/∂[itex]_{m}[/itex])[itex]^{2}[/itex] * σ[itex]_{m}[/itex][itex]^{2}[/itex] + (∂[itex]_{k}[/itex]/∂[itex]_{v}[/itex])[itex]^{2}[/itex] * σ[itex]_{v}[/itex][itex]^{2}[/itex]
    when you learn uncertainties a little more in depth, I think you will find it is much easier to use that with larger expressions.
    Last edited: Jan 10, 2014
  4. Jan 10, 2014 #3
    sorry I don't really know what that means though
  5. Jan 10, 2014 #4
  6. Jan 10, 2014 #5


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    Gold Member

    That's fine when uncertainties are given in terms of standard deviations. It might not be appropriate when given in terms of ±.
    If the lengths of two components to be manufactured have specs of ±1mm, and they are to be joined end to end, then the uncertainty in the total length is ±2mm. An engineer relying on the total uncertainty being only ±√2mm would soon be out of a job.
    A key issue is what is the definition of 'uncertainty' here. If it means standard deviation then you first have to convert the ± data to a standard deviation, and for that you need to know the distribution of the error. In particular, consider the case of measurements taken by eye against a graduated scale. The measurer will round to the nearest unit on the scale. The error therefore has a uniform distribution, ± half the scale unit size. The sum of two such measurements has a different distribution.
    jgray, unless you have been taught to use Panphobia's formula for such questions, I suggest just considering the extreme values for the energy that can arise from the ranges of possible values for mass and velocity.
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