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

I don't feel this is a misguided discussion at all. A torque requires an r vector and an F vector. It certainly doesn't matter what the location of those vectors happen to be. However, the vector r is not just picked out of the air. It is generated by two locations: the point where the torque is taken about, and either the point of application of the force or a point along the line of action of the force. Once we have used those points to define r it becomes like any other vector and has only direction and length and can be used with F to define a torque. But without those two original points, it has no meaning.

Bob at PC
________________________________________
From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Mallinckrodt [ajm@csupomona.edu]
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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