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Torque vs. Moment

by flatmaster
Tags: torque, torsion
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Dec13-08, 11:23 PM
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I was talking with an enginering student and arived that "moment" and "torque" mean basically the same thing. Are there any subdtlties to this definition other than the moment is the SUM of all torques at a point?

This being said, is there any other fundamental concepts that carry disparate names among different diciplines?
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I was talking with an enginering student and arived that "moment" and "torque" mean basically the same thing. Are there any subdtlties to this definition other than the moment is the SUM of all torques at a point?
Hi flatmaster!

i] "torque" means specifically moment of a force, but "moment" could mean moment of other things … but when talking about moment of a force, i think physicists use "torque" and "moment" interchangeably

ii] "torque" is sometimes used to describe a pair of equal-and-opposite (but not in-line) forces …

for example, if a brake is applied to one side of a flywheel, the combination of the friction force from the brake and the reaction force from the axle is a pair of forces which cancel out linearly, leaving only a torque !

iii] generally, "torque" tends to be used when there is an axle or pivot to be turned around, while "moment" tends to be used in essentially non-rotational situations, such as stress on a beam … as wikipedia says:
In the context of mechanical engineering, the terms "moment" and "torque" are not necessarily interchangeable; rather, one or the other may be preferred in a specific context. For example, "torque" is usually used to describe a rotational force down a shaft, for example a turning screw-driver, whereas "moment" is more often used to describe a bending force on a beam.
Dec28-08, 10:17 PM
P: 18
The way that it is used in civil and mechanical engineering:

Torsion is a specific type of moment. Torsion is simply a moment about the cross-section axis of the object. For example if you have a pipe or an I beam, torsion is a moment about the cross section of the pipe or the cross-section of the beam. Other moments such as bending moments from loading the beam are not called torques.

(I realize you could take a cross-section about any axis of the object, but in general I am talking about objects like a steel beam where the plane of what is considered the cross-section is pretty obvious)

It can also be explained as a series of shear forces going all around the cross section in the same direction.

The distinction between bending moments and torsion are made because usually one or the other controls the design and also the building codes will have different formulas and factors of safety for them. For example, a drive shaft is pretty much only torsion while maybe you are designing a beam with a small amount of torsion due to off centered loads. Since the bending moments control the design, you design for that first and then go back and check the torsion strength.

Dec30-08, 08:49 AM
P: 6
Torque vs. Moment

Usually, when you talk about torque it means that the vector for torque is on the object or within it, or that it's a single vector.

Moments are more broad in the sense that they can be placed anywhere. Wherever you want, and especially couples. Also, moments are related in my field (Civil Engineering) towards point loads or distributed loads. Distributed loads being the most common.

Thus, we have a moment diagram =). Mechanical Engineers use the idea to their advantage to make some calculations much more simple. Such as, when two forces intersect. And the force vectors usually intersect outside of the car. So we have a moment existing not on the object, but probably way outside of the object.

(example frames).

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