h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

In engineering (and physics!) a moment is a bending force. The moment of a force about an axis is the product of the force times the distance between where the force is applied and the axis.

Work, in engineering (and physics!!!), is the what a force does when is causes a displacement. So work is the product of the force and the displacement. If no displacement takes place, the force does no work.

Two minor corrections. Moment isn't a bending force, it is the turning /effect/ of a force and when ever you quote it you say wrt point A or whatever since you can take moments from anywhere

In cases of equilibrium the sum of all moments (clockwise and anticlockwise) should be zero.

Argh... I hate PH1 and M1!

Well, partially true.

Strictly speaking, a moment isn't a force, but it is generally referred to as one. And yes, moments are about an axis (not a point), as I stated (but it would seem to be a point when looked at in only 2 dimensions). And yes, in equilibrium, the sum of moments about an axis is zero.

Corrections again

Moments are about a point, line, plane or whatever fills the dimensions up. You can calculate moments about any point, line, plane, not just the pivot. Moments are the turning effect of a force exerted.

A moment is always about an axis. You're right that it is the turning effect of a force, but it is the turning effect about an axis. When you calculate moments about a point, what you are doing is calculating moments about an axis that goes through that point. My old Statics text from first year Engineering (which I took 24 years ago ) states it thusly:

The tendency of a force to make a rigid body rotate about an axis is measured by the moment of the force about that axis.

You seem to know more about moments than most people - are you an engineer too?

We have been specifically told off for saying that we calculate moments around an axis because that implies the pivot. We have to say around a line AB or a point A (for rods etc)

No... I am studying A level Physics and Maths Mechanics

Ah, I see. Well now you have something to tell your teachers! Really, it is only a matter of symantics - we all understand the concept. On a diagram, moments look like they are taken about a point since the paper is only 2-dimensional. But that point is really the end of an axis extending out of the paper (and into it, I suppose). After all, the moment is causing rotation in the plane of the paper. Rotation about a point could be out of the plane of the paper, in any plane whatsoever, since the point is presumably spherical. It is only unambiguous if it is about an axis. And you can't have a moment about a line, unless the line represents an axis.

partially right

you can calculate moments in spaces with more than three (four respectively, sorry albert...) dimensions. the equivalence for the axis then is nothting you can paint or display figurative.

Does anyone actually do that? Sounds way too complicated for me. As an engineer, I tend to like practical problems, in our 3 (all right, 4, Albert) dimensions.

calculating things in physics oder engineering sciences is always just a special case of more general concepts. in theoretical physics (or mathematics) you can find the generalized approach to such problems. the reason for doing such is to gain insight in correlations which you wouldn't get from a specialised approach.theoretically this empowers you to solve problems with just the knowledge of a few formulas (i.e if you understood the maxwell equations in all consequences you would be perfect in electrical engineering)

Of course, you're right. And engineers often go back to first principles too, when faced with something unusual. Fortunately, that hasn't happened to me lately (Actually, it does happen to me fairly regularaly - I work in a wind tunnel laboratory, where we do both research and consulting work, and I do some of both. It's a fairly limited field, though, and I can get my mind around most of the underlying principles).

since you are working on properties of gases or fliuds (i presume that from your statment with the wind tunnel) you are dealing with something even heisenberg (or schroedinger ? ah i forgot) didn't want to deal with. rumor has it that he said the theory of fluids (hydrodynamics) was much too complicated (nonlinear mathematics) to be a fruitful area of scientific interest .

True! Theoretical fluid dynamics is a pain. However, what we do gets around the problem. We make models and blow scaled wind at them and directly measure the loads (moments, for example ). So we do things in a more empirical than theoretical way. Recently, though, our lab has gotten into computational fluid dynamics, where, for some relatively simple problems, the equations can be worked out and loads determined using only a computer. This requires powerful computers and only relatively recently was possible, but at some point it will make our wind tunnels with their empirical methods obsolete. I don't see it happening too soon, though. Nature has created a complicated, chaotic thing in the wind that will defy theoretical computation for some time yet!