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defined and undefined quantities in physics



Bob says:

In first approaching force, I make it clear that Fnet = ma is a definition.

It seems this has raised questions.

I think a problem here is that, while the statement is true, it can be taken
in more than one way. Stated this way it is the case that it is a definition
of force, and empiry* justifies it as a useful one, since it turns out that
forces usefully combine as vector quantities using vector algebra. I believe
the standard introduction takes mass, length, and time as fundamental and
therefore undefinable. Acceleration is then defined in terms of length and
time, and finally force is defined in terms of mass and acceleration. It
could be done differently, however. If one chose force, length and time as
fundamental quantities then this equation would define mass instead. In
practice we now take mass, time and speed (of light) as fundamental, and the
choice is somewhat arbitrary.

and continues:

To me, physics is the study of nature and an attempt to model it.
Only use the model when it works, and look for a better one if it
doesn't. But always check the model and know its limitations.

Athem!**

Leigh

*empiry: observation and/or experiment, or the results thereof. You will
find this word in the dictionary, but not with precisely this meaning. A
word with this meaning is needed in the English language, so I have chosen
an appropriate disused word and am reconditioning it, sort of a recoinage.
If you will use it (and explain its meaning) you will aid my cause.

**athem!: you can say that again! This is a gender neutral etymologically
bankrupt non-religious PC ejaculation here meant entirely in jest; please
don't reuse it.