# moment for 3D model

#### ling233

in this case( involve 3D model) , it give the moment about the x , y , x' and y' axis .... can we find the moment about a specific point? (i.e. can we find the moment about the O ( centroid) ) ?

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#### ChipB

PHF Helper
. can we find the moment about the O ( centroid) ?
Sure. But it won't do you much good with respect to solving this problem though. Choosing to calculate moments through the x'- and y'-axes simplifies the problem nicely.

#### ling233

Sure. But it won't do you much good with respect to solving this problem though. Choosing to calculate moments through the x'- and y'-axes simplifies the problem nicely.
do u mean if i choose to find the moment about O , then i would have to find the moment about x , y , z axis about O ? in 3D question , if i choose to find the moment about a single point , then i would have to get the moment about the x, y, z of forces for the specific point ?

#### ChipB

PHF Helper
You may choose to calculate moments about any point that you want. You have a total of 6 equations to play with at any point (three for forces in x-, y-, and z-dimensions and three for moments about x-, y- and z-axes). So as long as there are no more than 6 unknowns you should be able to find a solution (if one exists) no matter which point you choose to use as O, and no matter which particular directions you choose for x-, y- and z-axes (as long as they are independent, or better yet orthogonal to each other). However, by using some judgment about selecting which point to use and which way the axes are oriented you can often reduce these 6 equations to a more manageable number, thus making the problem much easier to solve. For example in this problem by selecting axes x', y' and z' the 6 equations simplify to become just two.