kinematics language -- Re: kinematics, traditional or not

[Aaron Titus <titus@MAILAPS.ORG>]

Recently I've been observing our use of certain words in teaching
physics.  Some words are extremely confusing and I advocate not using
them.  Other words, we often used inconsistently.

> The problem arrises when you want to talk about negative accelerations.
> They are likely to see decceleration as a different conceptual thing,
> rather than the negative, or opposite of acceleration. There will also be
> a problem with negative velocities have positive accelerations.

I suggest never using the word "deceleration".  If one means, slowing
down, then I suggest saying "slowing down".  If one means, an
acceleration in the -x direction (or along whatever axis), then say
that.  If one means, an acceleration that is opposite to the
velocity, then say that.  Deceleration is often used with different
meanings. It's no wonder students are confused.

>  They need to see that when a ball rises the velocity decreases at the
> same rate that it increases as it falls, and conclude for themeselves in
> the end that the acceleration is the same in both cases.

If you define the +y-axis to be upward, the y-velocity is always
decreasing (i.e. dvy is negative for any given interval; slope of vy
vs. t graph is negative) even as the ball is rising.  However, the
magnitude of the y-velocity decreases as the ball rises and increases
as it falls.

The words "increasing" and "decreasing" can be confusing if one isn't
careful to distinguish between the magnitude of a quantity and the
quantity.  I use this example in my class: If your checking account
is in overdraft protection with a balance of -$400 and you deposit
$300, has your balance increased or decreased? <increased>  Has "how
much you owe the bank" increased or decreased? <decreased>


AT