Posted by: JJA () on Tue May 25 09:06:40 2004

jack, with your busy schedule and all, i think you overlooked a question i asked you recently...i'll repeat it:
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> upon reflection i think teacherman made a valid point...if a boxer delivers a blow to the opponent's face, is that torque?
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> i understand your point, which was that even though the bottom hand is "stationary" rather than actually moving in the opposite direction, this can result in two forces acting in opposition to each other...but is that torque?...if not, what do you call it?...standing on the ground creates two forces acting against each other, pushing your hand against the wall results in two forces acting in opposition, but i don't think any of us call that "torque".....

Hi,

I'll give you the precise answer here. One of the key concepts to remember is that torque is intimately tied to the center of mass of a body. This is one of the key conceptual difficulties involving torque. We'll get to some examples in a moment. With this in mind, torque causes a body to rotate about it's center of mass, while force causes the center of mass of a body to linearly accelerate.

The simplest example of a pure torque is a rod with a force applied at one end of the rod (perpindicular to rod), and a force in the opposite direction applied at the other end (also perpindicular to rod). In this example, the rod purely rotates about it's center of mass (the middle of the rod), but it doesn't translate.

The second example is a force applied at only one end of the rod,(perpindicular to the rod). In this example, although only a force is applied at the end of the rod, there is a net torque on the rod's center of mass, given by the product of half the rod length times the applied force. Thus, the rod rotates about it's center of mass. However, the center of mass rod also translates in the direction of the applied force. Thus a single force does apply torque about the rod's center of mass. It does not require two forces to apply torque about the center of mass of an object, although it does require two forces to apply a pure torque (i.e., no translation) about the center of mass of an object.

Let's take your example of a punch to the face. If the punch is directed through the center of mass of the head (i.e., approximately through the nose), there is no torque about the center of mass of the head and the head simply snaps back. However, if the punch is directed off center, say in the cheek, then the force of the punch would induce the head to rotate about the neck due to the net torque on the center of mass of the head.

Lastly, your wall example. To make it simpler, think of a wall standing by itself, no other support structure. If you directed the punch directly through the center of the wall, both horizontally and vertically, there would be no torque induced on the wall's center of mass. However, if you punched any other location on the wall, there would be torques induced about the center of mass of the wall in two axes on the wall, about the vertical axis of the wall, and the horizontal axis of the wall. Of course since the wall is bolted to the ground, the applied torque is directed into the ground through the wall. However, if the wall was hinged, like a door, punching the wall at any point other then the center of mass of the door would cause the wall to rotate, even though no torque is applied to the wall.

Sorry for the long explanation, but unfortunately that's what it takes to be precise. I hope this helps.

-JJA