OK, now we're in a position to understand how to make other
units. A good place to start is with the idea of density.
Density measures, well, how dense something is. If you take
a bunch of cotton candy, you'd say it has a low density because
it takes up a large volume, but doesn't have much mass. So
density should involve the ratio of mass to volume. We physicists
like Greek symbols to make all this stuff seem more sophisticated.
Here it goes: the density 
is defined as the ratio
of mass m to volume V.
The units of density are what? The units of mass over the units
of volume. What's the units of volume? Volume always involves
a length cubed. For example, the volume of a cube of side L
is 
. The units of volume are always the same, independent
of the shape of the object and are 
. So this means
that now we can express the units of density in terms of two fundamental
units as 
. To save writing and typing, just write
this as 
, 
much easier.
OK. That was simple enough. Now how about things with units
of length + mass? Is this a reasonable physical quantity?
The answer is no. You never get units like this occuring.
Length 
mass occurs a lot however. The reason is
that physics is ultimately about predicting what's going
on in the real world, not about fiddling around with
a lot of equations. If someone says, I'm 5feet and twenty
seconds tall, you'll think they're mad. To make a long story short,
adding different units together doesn't make sense.
Let's talk about what the units of force are. We'll see
later on that the force equals mass times acceleration. We'll
also see that acceleration has the units 
So we have
We can write this in SI units as 
.
