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Re: [Phys-l] bound vectors ... or not



John Mallinckrodt wrote... "If you want to get the physics right, you have to know exactly where
the force is applied, not merely the line along which it is applied."

I ask... are you sure?

John also wrote... "You also have to know if the point of application or the magnitude or
direction of the force changes with time."

I say... Of course, but I would reword this as: If the object is not static or if the force changes, you need to re-evaluate the calculation as the object changes position and/or the magnitude or direction of the force changes.

I came to this viewpoint after reading from the "statics" section of an engineering-physics text. But I don't see why it wouldn't apply to a dynamic situation if you step through the calculation incrementally over time, changing the object's position and the force magnitude/direction as necessary.

I am not sure I would describe this discussion as misguided. I think I have already learned some valuable things. Most of what I learned came from forcing myself to read an engineering physics book very carefully. But I would have never done that without the prompting of this thread. That book has been on my shelf for 21 years without me ever picking it up and actually studying it.

Just because we physicists do not typically view a force as having a location in space, that does not mean all professions view it that way. Engineers appear to view a force as having a line of action, and I believe that has given me some physics insight I did not possess prior to this thread.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu



-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Mallinckrodt
Sent: Tuesday, September 07, 2010 12:55 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] bound vectors ... or not

Mike Edmiston wrote:

I am not sure it is necessary to specify the "point of application"
of a force vector. I think it may be sufficient to specify a point
in space that lies on the "line of action" aka the "line of force"
of the force. The point of application could be the point that
fixes the force vector in space, but requiring that specific point
is probably overly restrictive. Once any point along the line of
action is specified, (along with the magnitude and direction of the
force vector itself), the specific point of application becomes
irrelevant.

If you want to get the physics right, you have to know exactly where
the force is applied, not merely the line along which it is applied.
You also have to know if the point of application or the magnitude or
direction of the force changes with time.

None of this, however, changes the fact that "force" is a vector and,
like all vectors, it has magnitude and direction, but not location.

Is it just me or does this thread seem particularly misguided to
anyone else?

John Mallinckrodt
Cal Poly Pomona
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