MadSci Network: Physics Query:

### Re: Do moment about a point and torque have resultant force acting on them?

Date: Thu Nov 30 10:10:44 2006
Posted By: Elizabeth DeBartolo, Faculty, Mechanical Engineering, Rochester Institute of Technology
Area of science: Physics
ID: 1162281628.Ph
Message:

Sean,

Actually, the other answer was the right answer, but I can see how this can be a little confusing. Maybe I can help clear things up...

Torque and Moment are both calculated by multiplying a distance by a force acting that distance away. The difference is in the interpretation of the resulting motion. Typically a torque implies a twist and a moment implies a bend.

For example, imagine you're holding three pieces of wood nailed together to form a capital "I". You are going to hold the I by the center of its base, flat out in front of you. If you have a friend come along and push down on the I right where you hold it, all that will happen will be for the whole I to shift down. You will need to apply a force equal and opposite to your friend's force to support the I.

Now let's consider five different scenarios, based on this picture:
:

1. If you have a friend come and push down on the top of the I, right in the middle, the I will bend - your friend has applied a Moment by offsetting the force a certain distance along the main axis of the I.
2. If your friend pushes down on the base of the I, where you're holding it, but at the end of one of the arms, the I will twist - your friend has applied a Torque by offsetting the force a certain distance off the main axis of the I.
3. Last, if your friend now starts pushing down on the top of the I, but at the end of one of the arms, the I will bend AND twist - your friend has applied a Moment and a Torque.

In any of these three cases, the force you apply to continue to keep the I from moving downward is equal and opposite to the force with which your friend pushes down!

A couple is a slightly different story, and it can be used to generate a Torque or a Moment. A couple is just a pair of equal and opposite forces that are offset from one another.

4. If you hold your wooden "I" from above at the base again, and have your friend push on the top two arms of the I, down on one side and up on the other, the I will still twist - meaning you've created a Torque. However, the two forces from your friend cancel out, and you don't need to supply force to keep the I from moving downward.
5. If you hold your I right in the center, and your friend pushes at the center top and bottom positions, up on one end and down on the other, the I will still bend - meaning you've created a Moment. However, again, you won't need to supply force to keep the I from moving downward.
With the applied couples, there is no resultant force, just a torque or moment, depending on whether the I is twisting or bending.

I hope this clears things up. Enjoy your Physics class!

Beth DeBartolo

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