Re: [Phys-l] the importance of F causing ma (and not vice versa)

["Folkerts, Timothy J" <FolkertsT@bartonccc.edu>]

John,

I don't quite buy into the terms of your challenge.  I see this as a
"learning spiral" issue.

> In particular, suppose I plan on having a research career, 
> calculating forces, measuring forces, calculating accelerations, 
> measuring accelerations, et cetera, all day every day.

1) As a child, I learn that forces make things move.
2) Later, I learn that pushing harder (say on my bike pedals) increases
my speed: F = mv.
3) In high school, I learn that forces are related to acceleration, not
speed.  I grasp the concept of force "causing" acceleration.
4) In college, I learn that I can't think of force and acceleration
separately.  F=ma is not a cause-effect relationship, but an intimate,
symmetric equation.  I come to know and believe F=ma.
5) In grad school, I come to know and believe that F=ma is wrong :-(  I
need to take into account relativity.  I need to take into account
quantum mechanics.
6)  I get a job at a particle accelerator calculating forces and
accelerations all day, and live happily ever after ;-)


If you are at Step 5 or 6, knowing that you want a job calculating
accelerations, then certainly you don't revert back to Step 3.  

But if you are still stuck at Step 2, you will never get to Step 6
without a few intermediate steps.  Heck, you probably don't even know
that people might have or want jobs calculating accelerations!  Perhaps
you could skip Step 3, but it provides a useful (in my mind anyway)
stepping-stone to the later levels of understanding.


> What will I be able to do with "force causes ma" that 
> I can't do with "force equals ma"?

To me, the question first needs to be "What will I be able to do with
'force causes ma' that I can't do with 'force causes mv'?"  Only when
that issue is resolved with the students can we proceed to the more
philosophical "force causes ma" vs "ma causes F" vs "F is equivalent to
ma" issues.

IMHO, "F causes ma" is a very helpful rung on a climb onward toward
understanding physics.


Tim Folkerts