Posted by: JJA (jjanagnost@att.net) on Sun May 29 14:48:33 2005

> >>> No, research10.html really isn't correct. Opposing forces are not required to produce torque. Please go to any standard college textbook (such as Beer and Johnston, 'Vector mechanics for engineers') and review the definition. Your definition is more consistent with the terms "pure moment" or "force couple" then the standard definition of torque.
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> In the example I cited, there are no opposing forces. The only force being applied to the rectangle is the upper force. ("Vectored inertia" is not a physics term).
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> However, I think I agree with your example of the pencil lying on the table. If you push on the pencil through its center of mass, the pencil will translate but not rotate. This isn't because of any opposing force (assuming a frictionless surface) but because you're pushing through the center of mass. If you push on the pencil at a point other than the center of mass, it will both rotate and translate.
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> Perhaps the confusion is the latter point. To cause a body to have pure rotation (and no translation) then opposing forces are required. Strictly speaking, however, this isn't how torque is defined. For example, in your lug nut example on research10.html, you use a 4 prong tire wrench to make your point. However, if you use a standard wrench you can unbolt the nut just fine by applying only a single force at a lever arm. Anyone will tell you that this method applies plenty of torque even though there is no opposing force being applied! <<<
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> Hi JJA
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> Let us cut to the bottom-line. I referred to the following definition of torque - “Torque is the result of two forces being applied to an object from opposing directions that cause the object to rotate about a point. “ – You appear to agree that opposing forces applied to the lug wrench does apply torque to the nut. In reference to the baseball swing, do you also agree that when the bottom-hand is being pulled in one direct (say forward) as the top-hand is being pulled in the opposite direction (rearward), torque is being applied to the bat?
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> Jack Mankin

Yes, I agree that torque is being applied to the bat in the example you stated.

The only reason I am arguing this is because we have now talked about several examples where rotation occurs but torque (according to a strict definition you provided in research10.html) does not occur. (The examples provided don't have an opposing force, so by your definition cannot be torque.) I don't think many people, scientists or lay people, could understand the statement that a body rotated but didn't have torque applied to it. It's totally unintuitive to most people's understanding of the word torque. That's why the physics texts delineate between a "force couple"/"pure moment" and "torque".

Sorry Ray about going round and round.

-JJA