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Old Aug 2nd 2008, 09:40 AM   #1
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Question about base unit of mass

Why the base unit of mass, according to the SI is the kilogram (kg) and not the gram (g)? As we have the meter for the base unit of length, we don't have the kilometer. I find it strange for the unit of mass. Maybe due to the fact that a kilogram is more common in our language than a gram. Is there a good reason to have chosen the kilogram?
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Old Aug 2nd 2008, 02:44 PM   #2
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Originally Posted by arbolis View Post
Why the base unit of mass, according to the SI is the kilogram (kg) and not the gram (g)? As we have the meter for the base unit of length, we don't have the kilometer. I find it strange for the unit of mass. Maybe due to the fact that a kilogram is more common in our language than a gram. Is there a good reason to have chosen the kilogram?
The decision of which unit to use as a base in any system is made by the rather practical method of the relative size of the numbers. Problems in one branch of Physics might use different typical sizes than others. For example, in Electricity and Magnetism we use one of a vareity of CGS unit systems where the base unit of energy is the erg. In Quantum Physics we tend to use the Heaviside-Lorentz system where the unit of energy is the MeV (1 million electron Volts.) One erg is a ridiculously large unit of energy to be using in Quantum Physics, so you can see the reasoning behind the new base unit.

I have made a project, off and on, of listing the different unit systems and what the units of different quantities are in these base units. There are some really interesting features in the CGS system. For example, capacitance is measured in cm! This is not as weird as it might sound: capacitance is a purely geometrical construction and the CGS system simply makes that connection clear.

My own personal preference would be that everyone use one chosen unit system to make things simpler and not have to mess with these unit conversions. But hey, who am I?

-Dan
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Old Aug 2nd 2008, 03:21 PM   #3
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Thanks for your answer.
My own personal preference would be that everyone use one chosen unit system to make things simpler and not have to mess with these unit conversions. But hey, who am I?
I agree but with nuance : It gets very confusing for example when we are talking about constants of springs which are generally expressed in dyn/cm or N/m, the conversion confuses me (even if I think that $\displaystyle 1N=10^5dyn$. I prefer to think about how to make the conversion rather than knowing it by memory, at least for now that I'm starting Physics I).
But if we tend to use only one unit system, we would make calculus larger than they are even if we need to convert units after all. It's easier to use light years than meters when we are talking about cosmology (oh, maybe not at the very beginning of our Universe).
Also I didn't know we were using different unit systems. I thought we were using the SI which contains all the derivate units from the basic's ones. (except in the USA). But finally it's maybe better in a way to keep more than one unit system, not to get that confused, but as you said sometimes it's better to use one unit system over another in certain branches of physics. So using one unit system would eradicate some unit conversions but it would load more the calculus. (I'm not really sure, but that's what I think).
EDIT : Finally I fully agree with you. If we use only one unit system, the calculus wouldn't get harder. For example we would use the pico gram, or pico meter which can be written as 10 pg. (10 pico grams). Instead of $\displaystyle 10^{-12} g$ as I thought it would be.
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Last edited by arbolis; Aug 2nd 2008 at 03:27 PM.
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