# Mechanics question - Forces & Moments

## Homework Statement

[PLAIN]http://img262.imageshack.us/img262/4343/scan22.png [Broken]

## The Attempt at a Solution

For the first question my attempt was:

[PLAIN]http://img255.imageshack.us/img255/2628/screenshot20110110at182.png [Broken]

But this is wrong.

The correct answer for part a was:

-59.7i - 159k N.m

Please give some guidance as to where I'm going wrong on the first one, and a starting point for the second one.

Thanks!

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## Answers and Replies

PhanthomJay
Science Advisor
Homework Helper
Gold Member
I find using M = r X F is rather difficult in 3D problems. Instead, try breaking up F into its x, y, and z components, and then use the "moment = force times perpendicular distance" approach, that is
Mx = Fy(z) + Fz(y)
My = Fx(z) + Fz(x)
Mz = Fx(y) + Fy(x)

where x y and z are the perpendicular distances from the line of action of the component force to the axis about which you are taking moments.

I find using M = r X F is rather difficult in 3D problems. Instead, try breaking up F into its x, y, and z components, and then use the "moment = force times perpendicular distance" approach, that is
Mx = Fy(z) + Fz(y)
My = Fx(z) + Fz(x)
Mz = Fx(y) + Fy(x)

where x y and z are the perpendicular distances from the line of action of the component force to the axis about which you are taking moments.

Thanks for your help, the problem is I'm being taught the M = r x F method and I'm generally used to it so I'd rather stick to it really!

Here's my working for the question, could you tell me where I'm going wrong on it if possible? I'm sure my r and F vectors must be wrong.

[PLAIN]http://img255.imageshack.us/img255/2628/screenshot20110110at182.png [Broken]

If not then I'll try your method, it's just that I'm not familiar with it and I'll probably end up getting more confused! Thank you.

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PhanthomJay
Science Advisor
Homework Helper
Gold Member
Well Ok I never use your method so I have to pass. I'll ask another helper to help, sorry.

For starters you're missing a component of your r vector (from A to C) its a 3D vector. If I were you I would define a vector from (0,0,0) to A (Ra) and from (0,0,0) to C (Rc) and then perform Rc-Ra, to get your r vector. Do the same for the Force vector.

P.S. Roomie, Phantom Jay's equations are simply an expansion of the cross product you're performing....nothing is confusing about it.