# Projections of Moments on a new Axis

tave
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

Homework Helper
welcome to pf!

hi tave! welcome to pf! 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)