Re: F=ma

[Rondo Jeffery <RJEFFERY@WEBER.EDU>]

Re Clarence Bennett's comments:

> > > Clarence Bennett <bennett@OAKLAND.EDU> 11/05/99 02:13PM >>>
> >
> > Or V=Ir -- potential "causes" current.
> >
> > >
> > Jim Green
>
> I would prefer V/R=I, to keep the conditions on the side of cause, and the
> result standing alone.
>
> (assuming a voltage source)
>
> Also,  How about  V/R => I, so to distinguish Causes from Equals.

This brings up something that I find students have great troubles with.  When they learn Ohm's Law as V = IR, they come to believe that if I = 0 then V = 0, even if the voltage they are trying to determine is not referenced to ground.  In electronics, I stress that Ohm's Law should really be thought of as *delta-V* = IR.  That way they can see that if both ends of the resistor are at the same potential in a circuit, the current through it is zero, and vice versa.  I would really like to see it taught this way at the intro level.  It would save a lot of grief when they get to junior electronics.  Of course, it also should be introduced as
I = (delta-V)/R, not the linearized way it is given in textbooks.

As for Newton's 2nd law, I agree 100% with those who argue for presenting it as
  a = (Fnet)/mass
Then, if you do write it in linearized form, Fnet = ma, as in solving free-body diagrams, students must get in the habit of writing all the forces on one side, and setting that equal to ma.  One of my kids had a teacher in high school who put in the FBDs an additional force which was called the 'acceration force.'  When I heard about that I about went into orbit.  It's no wonder the students were confused.

Rondo Jeffery
Weber State University
Ogden, UT 84408-2508
rjeffery@weber.edu