1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Finding uncertainty of a measurement

  1. Sep 22, 2016 #1
    1. The problem statement, all variables and given/known data
    What is your average reaction time? What is the uncertainty?

    2. Relevant equations
    To find time I used the following equation:
    D is measured in cm, hence the division by 100 (to get meters).
    9.81 is acceleration due to gravity (this comes from [itex] x = \frac{1}{2}at^2[/itex])
    While measuring D, I used a measuring stick with smallest unit 1cm. Therefor, uncertainty of D is [itex]\pm[/itex]0.05cm.
    My average time and distance are 15.48cm and 0.17s.
    3. The attempt at a solution
    I'm not certain what the best way to go about this is. So, the first thing I do is convert the absolute uncertainty to a relative uncertainty: [itex]\pm 0.05cm \longrightarrow \frac{0.05}{15.48} \times 100 = \pm 0.32[/itex]%.

    With a relative uncertainty, multiplication and division by a constant no longer matter, so far as my understanding goes. So, this simplifies down to a square root with a relative uncertainty. Therefore I just multiply my uncertainty by 0.5: [itex]\frac{1}{2} \times 0.32 = \pm 0.16[/itex]%.

    Is this the correct answer? Something about this whole process just didn't seem right to me, but I've never been good at uncertainties and whatnot.
  2. jcsd
  3. Sep 23, 2016 #2


    User Avatar

    Staff: Mentor

    That's not correct. Even though you are using relative uncertainties, the uncertainty on t depends on the absolute value of D. Have a look at http://www.rit.edu/~w-uphysi/uncertainties/Uncertaintiespart2.html [Broken]
    Last edited by a moderator: May 8, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted