Re: [Phys-l] Deceleration or Negative Acceleration

[John Mallinckrodt <ajm@csupomona.edu>]

John Denker wrote:

> Speed is a scalar. The derivative of speed is the _scalar_  
> acceleration. The scalar acceleration can be positive or negative.

In other words, "scalar acceleration" = d/dt | v_vec | (assuming
that "the derivative" is wrt time.)  Note that this is distinct, as JD
intends, from "magnitude of acceleration" = | d/dt v_vec |, a
scalar quantity that can *not* be negative.

I don't necessarily disagree with this, but I wasn't sure that I'd
ever heard anybody else use or refer to this definition so I did
some Googling on "scalar acceleration" and found, in the
first 40 hits

a) a lot of sloppy and/or irrelevant uses of the term,

b) a few who define it identically with "magnitude of
acceleration," and

c) three posts in which it is defined as above including:

  1) two previous posts on phys-l from JD

   https://carnot.physics.buffalo.edu/archives/2003/11_2003/ 
msg00495.html
   https://carnot.physics.buffalo.edu/archives/2003/11_2003/ 
msg00442.html

  2) one from mathworld

   http://mathworld.wolfram.com/Acceleration.html

So it would appear that JD has slim, but very good company!

> Deceleration is implicitly and intrinsically a scalar concept, and  
> corresponds to the negative of the scalar acceleration.

... which would agree with my own proposed definition.


A. JOHN "Slo" MALLINCKRODT
Lead Guitarist, Out-Laws of Physics
     http://outlawsofphysics.com
Professor of Physics (Ret'd), Cal Poly Pomona
     http://www.csupomona.edu/~ajm
Consulting Editor, AMERICAN JOURNAL of PHYSICS
     http://www.kzoo.edu/ajp