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Significant Figures: Dealing With Simple Calculations

  1. Jan 28, 2009 #1
    EDIT: I finished the assignment other than the two problems listed here, I got 91.66/100 but would love those last 8.44 points, I'll be checking in on this topic Thursday afternoon and might have figured it out by then but I am wary to waste my last attempt. So.. thats the update

    Ok throughout the years maybe I just haven't had a good explanation of significant figures teacher to teacher year to year and I do feel a bit pathetic posting such a simple "problem" but nonetheless here is my situation.

    I have five tries at each problem, and after looking up example after example and method after method I have gotten parts (a) and (b) correct but am left with (c) with one more attempt and (d) with two more attempts.

    My four attempts at (c) which were all wrong were: "26", "26.0", "25.6", "25." and with one more attempt at it I have no idea what they want from me as they give me no decent explanation to what I am doing wrong.

    My three attempts at (d) which were all wrong were: "0.03", ".03", and "0." with two more attempts, and I really cannot think of anything else to try and as I said it gives no decent explanation and I do not want to throw away another try until I can talk to someone to see what my problem is.


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

    Compute the following numbers, applying the significant figure rules adopted in this textbook.

    (a) 27.5^2
    (b) 32.9 * 21.3
    (c) 27.5 - 1.9
    (d) 1/33.5

    2. Relevant equations

    None. Significant figure guidelines. (Taken from assignment and pasted below.)

    Using Significant Figures in WebAssign
    Significant figures are one way of expressing uncertainty in measurement. The rules WebAssign uses to determine the number of significant figures in a number are shown in the examples below:

    1234 = 4 significant figures 5.0e2 = 2 significant figures
    500 = 1 significant figure 140E-001 = 2 significant figures
    500. = 3 significant figures 8.20000e3 = 6 significant figures
    13000 = 2 significant figures 101.001 = 6 significant figures
    2.000 = 4 significant figures 41003 = 5 significant figures

    As you can see, most numbers are fairly straightforward.
    To express a number like 1000 to 2 or 3 significant figures, you must use scientific notation.

    When you multiply or divide numbers, the result of your calculation has the same number of significant digits as the operand with the fewest number of significant figures. For example:

    1530 4.0 = 6100
    1530 (3 significant figures) 4.0 (2 significant figures) = 6100 (2 significant figures)

    When you add or subtract numbers, keep the fewest number of decimal places that are in all of the numbers. For example:

    2.46 + 6.1743 = 8.63
    2.46 (to the hundredths place) + 6.1743 (to the ten-thousandths place) = 8.63 (to the hundredths place)

    4580 - 411 = 4170
    4580 (to the tens place) - 411 (to the ones place) = 4170 (to the tens place)

    When you take the logarithm of a number, the number of decimal places in the result must be the same as the number of significant figures in the number you started with (Why is this?). For example:

    log10(27) = 1.43
    log10(27 two significant figures) = 1.43 (two decimal places)

    ln(0.026) = -3.65
    Keep in mind that certain numbers are considered to be absolute, such as the coefficients in the chemical formula below (for example, the 3).

    3 H2 + 2 C = C2H6

    Click here to close this window.


    3. The attempt at a solution


    I have five tries at each problem, and after looking up example after example and method after method I have gotten parts (a) and (b) correct but am left with (c) with one more attempt and (d) with two more attempts.

    My four attempts at (c) which were all wrong were: "26", "26.0", "25.6", "25." and with one more attempt at it I have no idea what they want from me as they give me no decent explanation to what I am doing wrong.

    My three attempts at (d) which were all wrong were: "0.03", ".03", and "0." with two more attempts, and I really cannot think of anything else to try and as I said it gives no decent explanation and I do not want to throw away another try until I can talk to someone to see what my problem is.
     
    Last edited: Jan 28, 2009
  2. jcsd
  3. Jan 28, 2009 #2
    I've continued to do the other problems given in the same homework assignment and none of the same rules seem to apply.

    Is it because they are conversions? IE: Convert kg/cm^3 to kg/m^3 (etc)
    And those numbers are known absolutely? Because I seem to be able to go to as many decimal places as I want on those problems and it accepts my answer, I was wary and now I just realize it accepts any # of decimals, I don't get it.. arg, when does it apply and when does it not?

    Anyway I'm pretty much done with the assignment other than the two problems with sig figs I posted above and it is not due until Friday night so hopefully one of you can help me before then.
     
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