Mechanics question - Forces & Moments

Click For Summary

Discussion Overview

The discussion revolves around a mechanics problem involving forces and moments in three-dimensional space. Participants are attempting to solve a homework question that requires calculating moments using different methods, specifically the cross product method and an alternative approach involving force components and perpendicular distances.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion over the use of the moment equation M = r x F in 3D problems and seeks guidance on their solution attempt.
  • Another participant suggests breaking the force into its x, y, and z components and using a different moment calculation approach involving perpendicular distances.
  • A participant indicates a preference for the cross product method, despite acknowledging the alternative method presented, and requests help identifying errors in their vectors.
  • One participant notes that the original poster is missing a component of their r vector and suggests defining vectors from a common origin to clarify the calculation.
  • There is a comment that the alternative equations provided are simply an expansion of the cross product method, implying that they are not inherently confusing.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the preferred method for solving the problem. Some advocate for the cross product method while others suggest an alternative approach, indicating a divergence in opinions on the best way to tackle the mechanics question.

Contextual Notes

There are indications of missing components in the vectors used, and participants express uncertainty about their calculations and methods. The discussion reflects varying levels of familiarity with the techniques involved in calculating moments in three dimensions.

Roomie
Messages
29
Reaction score
0

Homework Statement



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

The Attempt at a Solution



For the first question my attempt was:

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

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!
 
Last edited by a moderator:
Physics news on Phys.org
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.
 
PhanthomJay said:
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

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.
 
Last edited by a moderator:
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.
 

Similar threads

  • · Replies 40 ·
2
Replies
40
Views
8K
  • · Replies 1 ·
Replies
1
Views
3K
  • · Replies 1 ·
Replies
1
Views
4K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 5 ·
Replies
5
Views
5K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 16 ·
Replies
16
Views
4K
Replies
5
Views
3K
  • · Replies 10 ·
Replies
10
Views
3K
  • · Replies 2 ·
Replies
2
Views
2K