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Projections of Moments on a new Axis

  • Thread starter tave
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  • #1
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Hi all, this is my first post, so forgive me if this is in the wrong forum. (I believe it does belong here)

We are currently doing moments in physics and I have seen this in a few homework problems but can't seem to grasp it.

For example, in one question, we are given a new axis AA that is parallel to the y axis but is 150j and -100k (150, y, 100) (infinitely long in the y axis?) and the question wants the moment about that new axis AA.

My thinking is that I can take the projection about the y-axis or something along those line (I believe my thinking is completely off here)

M = u (r x f)
where r would be the position vector from the origin to anywhere on the AA axis.
F is the components of the force
u being the unit vector, and where I'm completely clueless.

Again, sorry for the lack of diagram or question, because this is more of a general question that applies to moments in general and axis's that are not touching the origin
 

Answers and Replies

  • #2
tiny-tim
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welcome to pf!

hi tave! welcome to pf! :smile:
M = u (r x f)
where r would be the position vector from the origin to anywhere on the AA axis.
F is the components of the force
u being the unit vector
no, r is the position vector from anywhere on the AA axis to anywhere on the line of the force

also, it should be u.(r x f), with a dot, so that it's a scalar

the moment (about one particular point on the AA axis) is actually the whole vector r x f,

what we call "the moment about the axis along AA" is really a scalar, the u-component of the whole moment: u.(r x f) …

(when r x f happens be along u, it makes no difference, of course, whether we regard u.(r x f) as a scalar or a vector: when it isn't along u, it's highly unnatural to call u.(r x f) a separate vector)
 

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