Re: BEFORE "Negotiating" a curve.

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

Hi John_D:

"It accelerates because it must" is not always wrong. In free fall;
for example, it is  an unusual way of saying "it obeys F=m*a".
The same with the phrase "unlimited resistance to any motion
other than pure rolling". Why should we use the terms such
"must" or "resist"? We want to describe a rolling rigid body
(wheel or sphere) in terms of Newton's three laws.  The
common term "resistance" is not part of vocabulary in mechanics.
You present elaboration clearly shows what was meant but this
was not obvious to me before. Thanks again for being helpful.
Ludwik Kowalski

*********************************************************
To understand is to find a satisfactory causal  relation.
To explain is to express that understanding.
To teach is to promote understanding.
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John Denker wrote:
<snip> ... </snip>

> > But that phrase a "wheel which offers unlimited resistance to any
> > motion other than pure rolling" is not really very different from the
> > phrase Brian used, "it rolls because it must".
>
> I don't understand what is your objection here.  I assume you accept the
> idea of a force of constraint;  after all, a book is _constrained_ to sit
> on top of the table;  the book/table force adjusts itself to match the
> weight;  the table offers unlimited resistance to further downward motion
> of the book.
>
> Is it that you reject the idea of a constraint between wheel and road?  Is
> it because the wheel is moving?  If so, try imagining the wheel as several
> dozen little "feet" arranged on spokes.  As the wheel rolls, each foot is
> stationary during the time it is in contact with the road.  It generates a
> force of constraint, providing "just enough" force to counteract any
> motion other than pure rolling.