# Homework Help: Really Stupid Conversions question

1. Dec 16, 2007

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

I am trying to get used to enginering prefixes and such. I would like to come up with a general way to reduce garbage composite units like Mg/ms into proper units.

Anybody have a general way. I know that my end result is to have no prefixes in denominator (except kg).

Should I break everthing up into scientific notation and tryto build the denominator to the base unit (in this case seconds) or what? How do you go about these.

I am sure after awhile I won't even think about it, but for now I would like a logical method if possible.

Thanks,
Casey

2. Dec 16, 2007

### mgb_phys

Do you mean just lose the multiplier prefix ( milli/micro/kilo etc) or do you mean reduce to base units mass/length/time etc ?

Removing the prefix is easy, just put the approriate powers of 10 and cancel.
Reducing to base units isn't always useful - no engineer wants a rate of pressure change in kg m^3 s^-3

3. Dec 16, 2007

The former. I have Mg/ms and I just want it to "look pretty".

So I should start with $$\frac{Mg}{ms}=\frac{1*10^6g}{1*10^{-3}}s=1*10^9\frac{g}{s}=\frac{1Gg}{s}$$

So I could say in general: establish the base unit to be used in the denominator.
Re-write denominator as such.
Re-write numerator in powers of 10 of its respctive base-unit.
Work from there.

This should work.

Thanks,
Casey

4. Dec 16, 2007

What about 1 mN/(kg*mu s) Should I keep kg in denominator since it is base unit of mass? Or change to something else?

$$1\frac{mN}{kg\cdot \mu s}=1\frac{kN}{kg\cdot s}$$ or is this poor form?

Sorry guys, but get ready for lots of stupid questions! I am taking a directed study over x-mas break in Statics. I am only meeting with the prof 5 or 6 times. The rest is on me (with PF help of course)

Casey

Last edited: Dec 16, 2007
5. Dec 17, 2007

### FredGarvin

You always want to get back to your base units. In your case kg, m and sec. All other units are going to be based on them. Unless you have special applications in which you already know the units will fall out for you.

When I come across problems with units like you have shown, the first thing I do is get rid of all of the prefixes, except for those on kg. Use those for all of your calculations and then convert back to prefixed units at the end. I find it too easy to make a mistake that will make you orders of magnitude off.

In your example above, make it:

$$1 \frac{mN}{kg*\mu s} = 1000 \frac{N}{kg*s} = 1000 \frac{m}{s^3}$$

The last simplification may not be a good thing to do if you need the extra units to simplify the units at the end. That would be up to you.