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Homework Help: Calculating percent error in a lab please help

  1. Mar 4, 2010 #1
    calculating percent error in a lab please help!!

    1. The problem statement, all variables and given/known data

    I have to calculate an experimental value for g using values from my lab. I got -45.9 as my value, and now i have to calculate the percent error

    2. Relevant equations

    a= g (m2 - m1/ m2+m1) was the equation i used

    actual-calculate/calculated x 100

    3. The attempt at a solution

    Would I use negatives or not?

    9.81- - 45.9 / - 45.9 x 100 = -121.4 %


    9.81-45.9 / - 45.9 x 100 = 78.6 %

    part b says derive the equation for a in the form a = F/m (f and m in terms of forces and messes in the atwood machine) but I have no idea what he is asking me to do

    thank you!
  2. jcsd
  3. Mar 5, 2010 #2


    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi wiccabean21! Welcome to PF! :smile:
    i] g = -45.9 is totally different from the correct figure :rolleyes:

    either you've applied the equation incorrectly, or (more likely) you need to do the experiment again :wink:

    ii] "error" means the likely error, not the actual error …

    decide how much error there is in measuring each of your input figures (for example, maybe there's a 1% error in measuring each mass), and then add them (or combine them in some other way) to get an overall error in the output figure. :smile:
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