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Re: [Phys-l] Deceleration or Negative Acceleration

Michael Edmiston wrote:

I disagree with John Mallinckrodt that it is reasonable to define
deceleration as - d/dt | v_vec |.

It is certainly your right to take issue with the proposed definition, but I can't understand how you could find it unreasonable. I continue to maintain that this is very precisely what "deceleration" refers to in common parlance. Do you really disagree?

As John said, this means positive deceleration is slowing down and negative
deceleration is speeding up. But this is not parallel usage to acceleration
(should I say it is not anti-parallel usage to acceleration).

I never implied or intended to imply otherwise. Given my druthers, I'd stamp out the word, but that's not an option. So, given that the word is here to stay, why not make use of it as a learning opportunity.

In common language deceleration and acceleration imply opposites.

Agreed.

But if John defines deceleration as he did, then deceleration does not mean the opposite of
acceleration, so I don't see any way this reduces the confusion.

My intent was not primarily to reduce confusion. My intent was to make clear that "deceleration" does not in any way, shape, or form equal "negative acceleration" (given the definition of acceleration in *physics.*) For example, an object with an acceleration of 10 m/ s^2, west has a "negative acceleration" of 10 m/s^2, east (a weird thing to specify!) and a completely undetermined "deceleration."

In common language, acceleration is speeding up and deceleration is slowing
down.... period.

I still agree!

In physics, acceleration is either speeding up or slowing down depending on
whether its sign is the same or opposite the sign of velocity.

This is simply not true, but I'm sure you just misspoke. In physics, acceleration is a vector; it doesn't have a "sign." Moreover, "acceleration" need not be "speeding up" or "slowing down."

In physics, deceleration is not used.

As I clearly implied.

But I hope all of this helps to make my viewpoint clearer.

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