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Homework Help: Unertainty In Acceleration

  1. Sep 28, 2011 #1
    1. The problem statement, all variables and given/known data
    I am doing a simple experiment using photogate timers and an elevated track to calculate velocity and acceleration. This experiment measures the time it takes for an index card attached to the top of a cart to pass through a photogate when going down an elevated track. By putting two photogates at two different places on the track we get two different time values and therefore two different velocity values. Both of which are calculted using the length of the card as the distance (the photogate timers track how long it takes the card to travel through each one)

    Long story short I have my two velocity values and a percent error for each. I got this percent error by adding the percent error in the distance measurement of the care, and the percent error of the time values collected. So I have two different velocities with two different percent error values. In order to calculate acceleration i have to subtract these two velocities and divide by the total time spent onthe track. This value i also have a percent error for. I know when taking a quotient you have to add the two percent errors together, but what should i do with the Two velocity errors, I have

    [(V2 (plus or minus its percent error))-(V1(plus or minus it's percent error))]/(time (plus or minus its percent error))

    How do i find the uncertainty/percent error in acceleration?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 28, 2011 #2
    a = {v[itex]_{2}[/itex] - v[itex]_{1}[/itex]}/t

    The numerator is a difference. Hence use rule for uncertainty in a difference to find the uncertainty in v[itex]_{2}[/itex] - v[itex]_{1}[/itex].
    Then use rule to find the uncertainty in a quotient.
  4. Sep 28, 2011 #3


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    Staff: Mentor

  5. Sep 28, 2011 #4
    thank you, my problem was that i had the two velocities and their Relative errors, i had to convert that to absolute error to complete the calculation in the numerator, then once that was complete convert that value to a relative error once again, then add the two, thank you very much!
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