🚀 go-pugleaf

"Donald E. Simanek" <dsimanek@EAGLE.LHUP.EDU> wrote:

>On Mon, 15 May 1995, Guntbert Reiter wrote:

>> I prefer: Forces cause acceleration (not necessarily movement)
>> while couples cause a torque (not a rotation).

>You've still not gotten it right:

>Net force produces acceleration (F = ma), while net torque produces
>angular acceleration: (T = I*alpha) where T is net torque and 'alpha' is
>angular acceleration.

So, where`s the difference? (Although I think that we're arguing
just for beeing as exact as possible, because the result  --
linear +  angular acceleration  -- is doubted by neither of us.)

>A couple is defined to be the torque produced by two *equal* size and
>oppositely directed forces with a displacement between their lines of
>action. Any *good* freshman college physics book, or an introductory
>engineering mechanics book will confirm these standard definitions and
>laws. The torque due to a couple is obviously Fd where F is the size of
>either one of the forces and d is the perpendicular displacement between
>them.

You're making contradictions in your paragraph: 
"A couple is defined to be the torque ..." vs. "The torque due to
a couple is...". Between these two point of view I prefer the
second: two forces with nonzero perpendicular displacement cause
a torque and this causes an angular acceleration.

>> be a linear acceleration of the CM. By Newton's laws (inertia...)
>> we know that from this acceleration the CM should "feel" (I just
>> can't remember the correct term) a corresponding force pointing
>> in the opposite direction. So these two forces build a couple and
>> produce a torque.

>The CM (a mathematically computed point) can't "feel" anything. And
>Newton's laws say nothing directly about the CM. Once you have *defined*

Again you took me wrong: I know, that "feel" is a _very_ wrong
term, but as English is not my language of daily use, I run out
of "technical" term now and then, and I thought it would be clear
from the context...

About CM: We consider this as the point where the effect of
mass-related forces is thought to ...(HELP: "touch" is wrong
again).

>I *assume* (though it's not at all clear from your paragraph above) that
>the 'corresponding force in the opposite direction' refers to the reaction
>force in Newton's third law. But, no, it can't, for the reaction force

Of course it can't. But regardless of different "counting
habits": You can express one law as: "acceleration and force are
proportional". => Accelerating the CM leads by inertia to a force
in the opposite direction....

>applied to a stick in space, reaction force does not *in any way* enter

yes, but inertia!

>That motion will be a combination of translation and
>rotation, a result of the applied force, and the torque associated with
>that applied force. 

So you try to convince me of my own opinion. Please accept my
apology for not being as clear as I wanted - language problems.

>If, and only if, that force were applied so that its line of action passed
>through the CM of the stick, then the stick would move in translation only,
>with no rotation.
Ack.

bye, bye
	Guntbert
------------------------------------------------
Guntbert Reiter : guntbert@borg-6.borg-graz.ac.at
HTL Weiz
Graz
Austria