Re: speed and velocity

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

But I agree with you, Merv, and with what I were saying. For a velocity
component there is only one thing to consider to decide about the
sign. For the acceleration component the direction of motion alone
is not sufficient. That is what confuses students. And, I must admit,
I also make sign errors, occasionally. When this happened I used to
say "this was deliberate, to see who is paying attention". But now I
often say "you see, even your old teacher is wrong sometimes, learn
from errors and try not to repeat them".

Mervin Koehlinger wrote:

> On Wed, 25 Nov 1998, Ludwik Kowalski wrote:
>
> > Positive and negative components of accelerations give my students
> > more trouble than problems related to signs of velocity components.
> > A v component is positive if an object moves along the axis and
> > negative otherwise. But for accelerations it is "sometimes one way
> > and sometimes another", even in one dimension.
>
> If I understand what you're saying, I have to disagree with you.  Positive
> and negative acceleration refers simply to direction, not whether the
> object is speeding up or slowing down. If an object is traveling in the
> positive direction and is slowing down that is negative acceleration. If
> an object is traveling in the negative direction and is speeding up, that
> is also negative acceleration.
>
> In general, if the acceleration is positive, the velocity vector is
> becoming more positive.  If the acceleration is negative, the velocity
> vector is becoming more negative.