Re: The sign of g

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

This is a one dimensional problem and as such fits well into the
following approach.

An object accelerates if it speeds up, slows down, or changes direction.
In one dimension, an object that speeds up has an acceleration in the
same direction as the motion while an object that slows down has an
acceleration opposite the direction of motion.  Now for the vertically
thrown ball, on the way up it is slowing down, therefore the
acceleration is opposite the direction of motion--downwards.  On the way
down the ball is speeding up, therefore the acceleration is in the
direction of motion--downwards.  The acceleration is always downwards.
[Now convince them it is still accelerating at the peak of the motion.]
You've defined DOWN as the negative direction in this problem.
Therefore since the acceleration is always down, it must always have a
negative value.  Of course you could (and should at times) define down
to be the positive direction.  The ball still accelerates downwards, but
now 'g' would carry a positive sign.

Rick

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Richard W. Tarara
Professor of Physics
Dept. of Chemistry & Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu
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> -----Original Message-----
> From: phys-l@lists.nau.edu: Forum for Physics Educators [mailto:PHYS-
> L@lists.nau.edu] On Behalf Of Tina Fanetti
> Sent: Tuesday, September 03, 2002 1:59 PM
> To: PHYS-L@lists.nau.edu
> Subject: The sign of g
>
> I know we have discussed this before I think, and I read about it in
Aarons..
> How do I explain to a student that when we choose up (+y) to be
increasing, we
> say that the sign of g is negative, regardless of whether the ball is
going up or going
> down.
>
> My students seem to get that when the object is going up, g is
negative.  They want
> to make the ball going down positive g.  But it stays negative because
of the choice
> of axis.
>
> I am at a loss because until I read Aarons I thought that it changed
signs too...
> Tina
>
> Tina Fanetti
> Physics Instructor
> Western Iowa Technical Community College
> 4647 Stone Ave
> Sioux City IA 51102
> 712-274-8733 ext 1429