Date: Thu Nov 30 10:10:44 2006
Posted By: Elizabeth DeBartolo, Faculty, Mechanical Engineering, Rochester Institute of Technology
Area of science: Physics
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
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:
With the applied couples, there is no resultant
force, just a torque or moment, depending on whether the I is twisting or
- 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
- 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.
- 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
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.
- 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.
- 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.
I hope this clears things up. Enjoy your Physics class!
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