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Re: (dv/dt) terminology: opportunity for improvement




A) In physics, we need a name for the quantity (dv/dt).
It's the same quantity that shows up as (F/m) in Newton's
second law. We like to use reference frames that lack
this quantity.

B) Alas, calling this quantity "acceleration" sometimes
causes problems for students. They think that acceleration
denotes an increase in speed, and that it is the opposite
of deceleration. This leads to wrong physics conceptions
in two ways:
-- Speed is a scalar, whereas velocity is a vector.
The quantity under discussion is a change in velocity,
and must not be confused with a change in speed.
-- We don't want two words (acceleration and deceleration)
when one word will do. A force in the forward direction
causes an increase in speed, while a force in the rearward
direction causes a decrease in speed, but we want to use
the same word to describe both of these cases (plus a
third case, namely sideways force).



While this goal is interesting, it is probably impossible because
acceleration has been the accepted term for a long time. Essentially we
have given it a "physics" meaning that is different from the common meaning.
Students need to learn to distinguish between physics meanings and common
meanings the same way they distinguish between conversational speech and
formal written speech.

The misconception that acceleration refers to both speeding up and slowing
down is the least of the many misconceptions that students must overcome.
The other misconceptions are probably more difficult and have nothing to do
with the name. Just to name a few they have to understand that +
acceleration is not speeding up and - not slowing down. They have to
distinguish between the idea of velocity and acceleration. They have to
understand that acceleration graphs do not look like velocity graphs. They
have to understand that acceleration does not go to zero when the velocity
is zero at the top of a ball's trajectory. They have to understand that the
acceleration has the same sign for a ball going up and coming down, rather
than different signs. The concept of acceleration as being a vector
quantity and not being a scalar is very difficult. Indeed the whole idea of
vector quantities and what they are is extremely difficult. Then of course
there are the whole class of misconceptions where students associate
velocity with force rather than acceleration with force.

In all I think the familiarity of the idea acceleration can be a plus in
terms of making them understand the idea, and I find that the other
misconceptions are much harder to treat. After all making deceleration just
a special case of acceleration is a simple definition. All the others are
much more conceptually difficult. One can use speeding up and slowing down
for changes in speed.

Incidentally a force in the forward direction only causes an increase in
speed if the car is going foreword. If causes a decrease in speed if the
car is going backward. See how easy it is to propagate a misconception!!!!

John M. Clement
Houston, TX

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.