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Homework Help: SI unit conversion help

  1. Jan 20, 2007 #1
    Ok, this isn't hard, I'm just thrown off by the question. From the book: "The quantity called mass density is mass per unit volume of a substance. Express the following in SI units:"

    a. Aluminum, 2.7*10^-3 kg/cm^3
    b. Alcohol, .81g/cm^3

    2. Relevant equations

    3. The attempt at a solution
    So do they want it just in g/kg?
    A. multiply by cubic centimeters per kg
    B. multiply by cubic centimeters per g
  2. jcsd
  3. Jan 21, 2007 #2
    SI units don't simply mean "metric". There are very specific metric quantities that are considered SI units. For example, the SI unit for temperature is the Kelvin. If you were given a Celsius measurement, you'd have to convert that to Kelvin if you wanted to express the temperature in an SI unit.

    You can see which units are SI units from this link:


    For example, in part a of your question, you're given kg and cm^3. One of those is SI and the other isn't.
  4. Jan 21, 2007 #3


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    Science Advisor

    SI units are also sometimes referred to as "MKS" units- the fundamental units of length mass and time are "Meter", "Kilogram", and "Second"- unit of force is the Newton, energy the Joule, etc. This is as opposed to the "cgs" system in which the fundamental units of length, mass, and time are "centimeter", "gram", and "second"- the unit of force is the dyne, energy the erg, etc.

    In any case, your first measurement is 2.7*10^-3 kg/cm^3 which is in neither system. You need to convert grams to kilograms. The second measurement is .81g/cm^3 which is in cgs. You need to convert grams to kilograms to grams and centimeters to meters.

    Use the "unit fractions" 1 kg/1000 g and 100 cm/ 1 m.
  5. Jan 21, 2007 #4
    The first measurement was already in kg. :-)
  6. Jan 21, 2007 #5
    The first one is already in Kg, so wouldn't I just convert the cm^3 to m^3?
    The second one needs to be converted to kg, then cm^3 to m^3.
    Last edited: Jan 21, 2007
  7. Jan 21, 2007 #6
    Yup! :-)
  8. Jan 21, 2007 #7
    m^3 is an SI unit still, right?
  9. Jan 22, 2007 #8
    Just imagine how odd it would be if you had two lengths expressed in SI units. You went to go multiply them together to find an area and you found that your area was not in SI units.

    I think we'd all go back to using the English system...