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: When working out the Uncertainties, what to do with the constants?

  1. Feb 17, 2010 #1
    Q=mcT, variables with uncertainties are m and T.
    If it were only Q=mT, the %uncertainty of Q would be %uncertainty of m + % uncertainty of T.

    But c=constant (no uncertainties), so what is the uncertainty of Q when Q=mcT? Do you just multiply the constant to the %uncertainties of m and T?
    What generally happens to constants when working out uncertainties?
  2. jcsd
  3. Feb 17, 2010 #2
    You would normally ignore it, as the value of the constant found in tables would have a very small uncertainty.
    Any uncertainty in the constant would need to be of the same order of magnitude as your experimental uncertainty for it to be necessary to include it.
    In your example, if it was water you were using, the value of c can be found from tables to be 4186 J/kg. This is presumably accurate to over one part in 4000. Much more than your readings.
    If your experiment used data that was more accurate than this, you would need to get the value of c to even greater accuracy.
  4. Feb 17, 2010 #3
    Oh ok I see...
    so eg. v=2as,
    then would I need to multiply the 2 to the uncertainties of a and s, or would I ignore it like you said?
  5. Feb 17, 2010 #4
    In that formula, just add the % uncertainties in the things you measure. Ignore the 2.
    It is the uncertainty in the measured values that you need to account for.
  6. Feb 17, 2010 #5
    ok, thanks so much!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook