Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about AMU & Temp. Conversions

  1. Mar 26, 2005 #1
    So... I'm wondering what the measurement of an AMU (atomic mass unit) is... I mean.. I've heard it's really the weight of a proton but then I'm told the proton's weight is actually 1.0073 amu.. so what is the amu based on then?

    btw: how do you pronounce a decimal number such as ... .0007 or .0000009?



    I was reviewing diff temp conversion and the "formulas" for them... I noticed something that kind of made me stop for a second...

    (F - 32ºF) x (5ºC/9ºF) = C

    Now.. knowing how both temps are the same at -40º, I worked it out and it doesnt quite work when it's done like this:

    -40 - 32 = -72

    now take 5/9 = .555555etc...

    that means it cant possibly be -40 but instead around 39.999 or something...

    Now.. if you instead cross multiple and simplify the 72 into an 8 and the 9 into a 1 then you get:

    -8 x 5/1 = -40

    My problem clearly is mathematical... so I'm wondering what I'm doing wrong.. and why with the two "valid" methods there is a difference, small but there's one.

    I hope it's not some theory of how .999999999999999999999 is technically one.
  2. jcsd
  3. Mar 26, 2005 #2


    User Avatar
    Science Advisor
    Gold Member


    Your first question is about carbon's 12-based amu system. If you look for carbon, you'll see that it is 12.00 amu, where its previous partner, oxygen, is 15.999 (formerly it was used as a standard to be 16.00).

    English is not my native language, so I don't know how to pronounce it. However, very small numbers like 0.000000000007 or 0.0000000000009 could be expressed by scientific means; the former becomes 7E-12 (or 7x10-12), and the latter becomes 9E-13 (or 9x10-13).

    Fahrenheit-Celsius conversion is very simple; the crucial thing here is that fahrenheit scale consists of 180 parts, while celsius has 100. Melting of ice is 0°C in celsius, but 32°F in fahrenheit, whereas boiling of water is 100°C in celsius, while 180°F in fahrenheit.

    If you want to convert a temperature given in Fahrenheit to Celsius, simply subtract 32 from the number, multiply by 100, and finally divide to 180. The ratio 5/9 is actually 100/180.

    I haven't heard of "same temperatures in both scales", so let's try it:

    [tex]x=\frac {(x-32)*5}{9}; 9x=5x-160; 4x=-160; x=-40[/tex]
  4. Mar 26, 2005 #3


    User Avatar

    Staff: Mentor

    Lasciate ogni speranza :)

    Chemical calculators for labs and education
    BATE - pH calculations, titration curves, hydrolisis
  5. Mar 26, 2005 #4


    User Avatar
    Science Advisor
    Homework Helper

    You should always the modern fraction notation and not use decimal notation in intermediary steps,unless u use the [itex] \approx,\simeq [/itex] signs...

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook