1. Not finding help here? Sign up for a free 30min 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!

Percentage Uncertainties Question

  1. Sep 24, 2016 #1
    1. The problem statement, all variables and given/known data
    Been given a problem where a force (F) is applied on a circular area (m2). If the percentage uncertainty of the force is 8% and the percentage uncertainty of the radius is 3%, what is the percentage uncertainty of the obtained value pressure?

    2. Relevant equations
    I know that pressure equals force over area and to find the area it is πr2. Anyway to work the percentage out?

    3. The attempt at a solution
    I assume you don't just 'add' the percentage uncertainties in this case. So far see that 8% over π3%2 would be the equation to work this out.
     
  2. jcsd
  3. Sep 24, 2016 #2

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    What does a percentage uncertainty actually mean?
     
  4. Sep 24, 2016 #3
    It's the absolute uncertainty expressed as a percentage. So how 'large' the uncertainty of a measurement. If it is just the measurements would it be 11%? As that is what's being 'measured' and not calculated?
     
  5. Sep 24, 2016 #4

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    You're still not thinking about what it actually means. For example, if a force is 100N with a percentage uncertainty of 8%, could that force be, say, 150N? Or, 62N?
     
  6. Sep 24, 2016 #5
    That force could be 92N or 108N...
    Force = 100 +- 8% N
     
  7. Sep 24, 2016 #6

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    So, you're saying the force could have a maximum value of 108N and a minimum value of 92N.

    And, the radius?
     
  8. Sep 24, 2016 #7
    I understand that there is a range of values that the area 'could' be, ranging from the area -8% to area +8%...
     
  9. Sep 24, 2016 #8

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    It's getting difficult to thing of a hint without actually telling you the answer! But, let's try:

    If the force is 100N (with 8% uncertainty) and the radius is 1m (with 3% uncertainty), could the pressure be 50N/m^2?
     
  10. Sep 24, 2016 #9
    Well from your example, the answer to the equation would be 100 over pi x 12? Then +- the percentage uncertainties added. So 31.8 +- 11% which means the answer is ranging between 28 and 35 Pa. Or would it be like considering bounds in maths, the maximum and minimum possible values for pressure with the equation?

    Sorry if I'm not understanding this as easily as most would.
     
  11. Sep 24, 2016 #10

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Why would the uncertainties add? Why not do a calculation with the maximum possible force and minimum possible area? And vice versa.
     
  12. Sep 24, 2016 #11
    So to answer that question it is like considering bounds then. Thanks.
     
  13. Sep 24, 2016 #12

    PeroK

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The answer to the question lies in calculations. And, yes, the maximun possible pressure must be the maximum possible force divided by the minimum possible area.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Percentage Uncertainties Question
  1. Percentage uncertainty (Replies: 4)

  2. Percentage uncertainty (Replies: 4)

  3. Percentage Uncertainty (Replies: 2)

  4. Percentage Uncertainty (Replies: 7)

Loading...