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Re: Questions on Neg. and Pos. Acceleration

At the risk of beating this to death, I want to emphasize the main point
here.

The choice of positive/negative labels is totally arbitrary. In one
situation we may choose up as positive, in another down as positive.
Therefore a _negative_ acceleration means ONLY that it is pointed in the
chosen negative direction.

Students DO seem to come to college with a built in sign convention from
most HS programs. All I have to do is ask what the value is for the
acceleration due to gravity and I get a chorus of "MINUS 9.8" with a few
additions of "meters per second squared". I immediately object. It then
takes considerable time to wean them (not 100% successfully) from this
notion and get them to adopt the 9.8 m/s^2 DOWNWARD description for 'g'.

For conceptual purposes I find that a ball thrown vertically upwards
provides ample fruit for most of a semester's pondering of kinematics,
dynamics, energetics, etc. It is certainly an example where we have the
ball both slowing down and speeding up despite the acceleration being in a
constant direction. Of course we all know the acceleration does go to zero
at the top of the motion. ;-)

Rick

*******************************************************
Richard W. Tarara
Department of Chemistry & Physics
Notre Dame, IN 46556
219-284-4664
rtarara@saintmarys.edu

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see: www.saintmarys.edu/~rtarara/ for details

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----- Original Message -----
From: jmanor <jmanor@REMC4.K12.MI.US>
To: <PHYS-L@lists.nau.edu>
Sent: Thursday, September 23, 1999 6:35 AM
Subject: Questions on Neg. and Pos. Acceleration


Is it possible for an object to have a negative acceleration while
increasing in speed? And also can you explain if the velocity of an
object be negative when its acceleration is positive and vice versa?