Fundamental quantities are numbers that we need to describe the
world around us, which we cannot express in terms of "simpler," more
basic quantities. I'll give a few examples.
My weight is not a fundamental quantity, because it depends on how
much stuff makes up my body. A good approximation to my weight is just
the sum of the weights of all of the protons, neutrons, and electrons
which make me up. If I could count those, I could calculate my weight
in terms of the masses of the proton, the neutron, and the electron
(and I'd need the local strength of gravity to get my weight once I
know my mass). The mass of a proton is more fundamental, although we
know now that protons are made up of smaller pieces (quarks and
gluons). People are actively trying to refine calculations of the
proton mass in terms of the stuff inside a proton and their
The electron does not appear to have any pieces inside of it, and
so as far as we know, the electron mass and the electron's charge are
fundamental quantities. The strength of gravity, which scales with
Newton's gravitational constant, is also a fundamental quantity. The
speed of light is a fundamental quantity (although these days the speed
of light really is used as a definition of how long a meter is).
Planck's constant, which governs how big atoms are and how much energy
photons have at given wavelengths, is a fundamental quantity.
We strive as physicists to have as few fundamental quantities
around as possible. Being able to calculate other necessary quantities
in terms of those few we don't know how to compute means we understand
more about nature.
Tensile stress is just the amount of force per unit area when an
object is pulled on opposite sides. Tensile strain is the amount the
object changes shape in response to this stress (objects will stretch
when pulled). Strain doesn't have any dimensions because it represents
fractional changes in the length. If the strain is 10%, then all pieces
of the object getting pulled on lengthen in one direction by 10%.
(published on 10/22/2007)