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: Simple unit convention question

  1. Jan 15, 2010 #1
    This is just a quick question about the convention used to write units. My assumption is that you wouldn't leave spaces between different units, but i could see this could lead to potential confusion. should i write:

    km s-1 Mpc-1

    I guess it's normally clear from what you're talking about and i don't think there are any letters used for more than one unit. Just want to check the convention. I assume it is without spaces?

    EDIT: there is a case where letters are used more than one thing, ms-1 could be per micro-second or metres per second. Guess it'd be obvious in context though?
  2. jcsd
  3. Jan 15, 2010 #2
    I have seen units written in numerous different ways.

    This couldn't be mistaken for micro units because the correct symbol for micro units would be

    the greek symbol mu (my LaTex commands aren't working properly) would be placed immediately before the m.

    For illustration purposes let u stands for mu then

    ums^-1 would stand for micro-meters per second. (Note: micro-meters is refered to as microns.)


  4. Jan 15, 2010 #3

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    In this case, I would use km/(s Mpc). When written with scientific notation, I prefer a non-breaking thin whitespace between units.

    You already mentioned one: ms-1. Does this denote a velocity (meters per second) or an inverse time constant (per millisecond). Writing m/s, m⋅s-1, or m s-1 leaves no doubt.

    Even with the spacing rules, there remains some ambiguity. For example, what does "1 as" denote: one arcsecond (a smallish angular displacement), or one attosecond (an extremely small time measurement)?

    The rules are (see page 39 of http://physics.nist.gov/Pubs/SP330/sp330.pdf)
    In forming products and quotients of unit symbols the normal rules of algebraic multiplication or division apply. Multiplication must be indicated by a space or a half-high (centered) dot (⋅), since otherwise some prefixes could be misinterpreted as a unit symbol. Division is indicated by a horizontal line, by a solidus (oblique stroke, /) or by negative exponents. When several unit symbols are combined, care should be taken to avoid ambiguities, for example by using brackets or negative exponents. A solidus must not be used more than once in a given expression without brackets to remove ambiguities.​
  5. Jan 15, 2010 #4
    sorry CFDFEAGURU i did mean milli honest - my brain was behind my fingers!

    Thanks for that, it seems to be the conventional way, though i do find the solidus and brackets approach rather less clear than negative indexing. I think i may be alone in that...
    when you say "non-breaking" do you mean stopping it going over to the next line of a report? in that case would i be best to just remove the whitespace?
  6. Jan 15, 2010 #5

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Yes. The text on both sides of a non-breaking whitespace must appear on the same line.

    No. You would be best to use a non-breaking whitespace.

    Any decent quality word processor (e.g., Word, WordPerfect), document processing package (e.g., TeX, LaTeX) or markup system (html) will provide such a capability.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook