Uncertainty

• Audi

  12 Jan 11, 17:52

  Some basic doubts to clarify.

  Let's say I want to find the uncertainty of this displacement which takes on this formula s=0.5at^2, do i INCLUDE the 0.5 in my uncertainty equation?

  I read through the rules today and found this:

  If a reading is multiplied by a constant k, the uncertainty is also multiplied by k.

  z = kx the uncertainty will be uncertainty z = k (uncertainty x)

  so I saw ANOTHER example, which is p = pi r^2 R/ L (where p is resistivity and L the length of wire)

  when they find the uncertainty of p, pi wasnt included in the equation. So i am wondering whether i should include 0.5 when finding the uncertainty of s = 0.5at^2. I still dont get why pi isnt included in finding the uncertainty of p.

  Thanks!

• eagle

  12 Jan 11, 18:39

  Your question is very general. Pls show an example working. You mentioned seeing examples, but I wouldn't know exactly what example you saw, what question was it, and what your workings and problems are.

   

  Please do note that there's a difference between finding uncertainty and percentage uncertainty. I believe you were doing the latter.

• wee_ws

  13 Jan 11, 08:45

  Hi,

    Given s = 0.5 at^2.

    Then we have 

    (delta s) / (s_0) = (delta a) / (a_0) + 2 (delta t) / (t_0),

    where s_0, a_0, and t_0 are given values of s, a, and t respectively.

    Constants are not considered by the fact that no certainty arises from a value that is always fixed.

    Thanks.

  Cheers,
  Wen Shih 

• Audi

  13 Jan 11, 22:34

  Originally posted by wee_ws:

  Hi,

    Given s = 0.5 at^2.

    Then we have 

    (delta s) / (s_0) = (delta a) / (a_0) + 2 (delta t) / (t_0),

    where s_0, a_0, and t_0 are given values of s, a, and t respectively.

    Constants are not considered by the fact that no certainty arises from a value that is always fixed.

    Thanks.

  Cheers,
  Wen Shih 

  Thanks all, I already knew how to solve this question :)

  I do know constants are not considered but how about cases which abide by this rule:

  For z = kx, the uncertainty will be uncertainty z = k (uncertainty x)

  In this case, we do multiply the constant.

  Thanks!

• wee_ws

  14 Jan 11, 06:35

  Hi,

    Which book or notes did you refer to?

    I have not come across this rule in my IB Physics text or in JJC lecture notes. Thanks.

  Cheers,
  Wen Shih