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I know that the direction of torque is along the axis of rotation, but would it be acceptable to say, for example considering a vertical thin rod in the x-y plane with a force acting on the bottom end towards the left, 'it is a clockwise torque about the centre of mass' (even though strictly speaking the torque's direction is into the page?)

It just seems to me easier to describe what's happening that way.

binaryfunt
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  • In fact when you do statics, you start by counting up all the resulting torques as +CCW and -CW, so just saying clockwise automatically means a negative value. Only in statics though, because the sum of moments is the same regardless of the point of reference. In dynamics, you must choose torques about the center of mass, always. – John Alexiou Apr 16 '14 at 13:04
  • I would usually add "as seen from..." but in your case it seems to be fine. – Floris Apr 16 '14 at 14:56
  • @ja72 I'm a little confused when you say, 'In dynamics, you must choose torques about the centre of mass, always.' What about if the rod is hinged about the other end? – binaryfunt Apr 16 '14 at 15:09

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In the situation you gave, it's immediately clear what is meant, and there's no possibility for misinterpretation, so yes, it's perfectly acceptable. (Remember that torque is mathematically defined as a vector for convenience, but the direction of that vector isn't really physical.)

The only issue I can see with that is that as you leave the simple two-dimensional world and start looking at more complex problems, you've quickly need to abandon that way of describing things, since "clockwise" and "counter-clockwise" are a lot harder to keep track of.

Scott Lawrence
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  • Thanks. I presume the same would apply to other rotational quantities such as angular velocity (actually, in my lecture notes, sometimes the diagrams do have a curved arrow labelled with omega, when I think about it) – binaryfunt Apr 16 '14 at 13:47
  • Compare to http://physics.stackexchange.com/questions/71455/what-does-clockwise-mean-exactly. Just because you are perfectly clear on what you mean does not imply that your listener is perfectly clear on what you mean. In order for "clockwise" to be clear you have to be have agreed on a "forward" first. – dmckee --- ex-moderator kitten Apr 16 '14 at 13:48
  • Yeah, it depends on the situation I guess. In my example I don't really see how someone could misinterpret it as a clockwise torque from the other side of the x-y plane – binaryfunt Apr 16 '14 at 13:55
  • Yes, that's why it's clear here. Pedagogically, I'd avoid it to avoid issues when adding a third dimension. – Scott Lawrence Apr 16 '14 at 15:36