Re: Newton's 3rd law? was Re: inertial forces (definition)

[Robert Carlson <Raacc@AOL.COM>]

In a message dated 10/24/99 3:57:26 PM Central Daylight Time,
ajmallinckro@CSUPOMONA.EDU writes:

> Arons speaks about these two as well as other different meanings of the
>  "equals sign" in his "Guide to Introductory Physics Teaching." The
>  confusion caused in the case of "Centripetal force = mv^2/r" is, I think,
>  pretty well acknowledged by most teachers.  Perhaps less so is the case of
>  "impulse = delta p" and "work = delta K."  In these cases the words
>  impulse and work are important and useful; they can't and shouldn't be
>  avoided.  But students don't often grasp the important difference between
>  writing "W = Fd" or "I = Ft" (which are *merely* definitions) and writing
>  "W = delta K" or "I = delta p" (which are important expressions of
>  physical relationships between separately defined quantities).  Students
>  will be a step or two ahead if they understand that they are not doing
>  physics when they use the former equations.  And lest anyone misinterpret
>  me here, I certainly don't mean to imply that the former equations should
>  not be used!

John has hit on an important word here, in my opinion, and that is
relationship.  I feel this word has been underused, simply because we present
so much material in the form of equations and not graphs.  For example, W =
Fd is the area of the F versus d graph.  Examining the area under the graph
results in a relationship between F, d, and W.  I = Ft is the area of the F
versus t graph.  Examining the area under the graph results in a relationship
between F, t, and I.  I believe, this is partially due to students not being
exposed to real data and the analysis of this data using graphs.  For
example, define Fnet = ma and a = dv/dt.  Then give your students the a v
versus t graph with three different slopes for a given mass and ask them to
find the net forces.

Bob Carlson