Re: g

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

Michael Edmiston wrote (in part):

> I think the original problem stems from the common feeling among students
> that positive acceleration means "speeding up" and negative acceleration
> means "slowing down."  This preconceived notion is strong and for some
> students I never succeed at getting them beyond it.  ....

It is interesting to speculate about origins of misconceptions.
Acceleration as a quantity in m/s^2 is probably never used
before students start learning physics. In fact the idea that
"positive acceleration means speeding up and negative means
slowing down" is a correct for problems in which velocity is
positive (which means the object moves toward the positive
end of the reference axis).

How can I support this idea? By making sure that the reference
axis is chosen to coincide with the direction of motion. A
problem with a rock which first moves up and then move down
can be solved in two steps, first with the axis up (to find out
where v becomes zero) and second with the axis down (to
answer a final question).

I am not suggesting that this simple approach should be
supported; I am only saying that it can be supported. It takes
"a giant step" to realize that the reversal of the axis (to match
the sign of v) is not necessary. But by doing this we must
give up the simple idea about "negative a implies slowing
down". In the context of new reality (allowing v to become
negative) the idea which was OK becomes a misconception.

How does this differ from situations in which Newtonian
dynamics becomes a misconception? First we teach it as
"always correct", then we say it is correct only if ..."
Ludwik Kowalski