# Homework Help: 3D moment

1. Oct 20, 2008

### ricky_fusion

1. The problem statement, all variables and given/known dataH
Hi, actually I don't know maybe this can be called as homework or just lesson
https://ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=st&chap_sec=04.2&page=case_sol
All data are explanation in this book.

Does anybody can explain me how to solve the problem in detail way?
2. Relevant equations
all equation are explanation too in the link above.

3. The attempt at a solution

Best Regards,
Ricky
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Oct 21, 2008

### alphysicist

Hi ricky fusion,

3. Oct 21, 2008

### ricky_fusion

I am sorry for not complite explanation, but I have try again to solve the problem and I have known the way to solve the problem. But if you mind to help he to open the next tab (Example tab), I still confuce find the way likes as he solve that problem.My problem is How to calculate the distance that perpendicular to the Force. I have try to used the right Hand method, but still can't find the distance. As I know from the lesson, that in 3D moment the distance must perpendicular with the force and to find the moment "try used the right hand method". Am I wrong?

4. Oct 21, 2008

### ricky_fusion

Oya, I am forget to explain that I won't see the way to solve the problem before I find the answer first. Because my objective are understand about the principle of 3D moment and understand to solve the 3D moment problem. For information, I don't get a school and my way to get knowledge just used Forum and some ecourse that free to read/studied. I am sorry if my language is too bad.

5. Oct 21, 2008

### alphysicist

I don't think the right hand rule will give you the distance here. Once you have found the lever arm and the force, then you can use the right hand rule to find the direction of the moment (direction of the purple arrow).

To find the distance it would be a matter of using the Pythagorean theorem. For example, to find the moment about Joint C, you are given that Joint C is 15cm below the line of force (in the y direction) and 45cm to the right of the line of force (in the x direction). The Pythagorean theorem applied to this:

d^2 = (15cm)^2 + (45cm)^2

can be solved to find d.

So can you see that the distances here are 15cm and 45cm for Joint C, from what they give you on the third picture of this page:

https://ecourses.ou.edu/cgi-bin/ebook.cgi?doc=&topic=st&chap_sec=04.2&page=case_intro

6. Oct 22, 2008

### ricky_fusion

Can you explain in more simple word or any reference??
Because now I am confucing about the concept of 3D moment.

7. Oct 22, 2008

### alphysicist

An important thing to remember is that to calculate the moment about point C, all you are given is point C and the line of force (the line through the red arrow). What you have to determine is the smallest distance (perpendicular distance) between that line and the point C. That smallest distance to the line of force is d, and is what you have to find in each case.

I noticed that you were getting this problem from the section "Moment 3D scalar". At the end of that it mentions that the next section is "Moment 3D vector" which it claims greatly simplifies the process. Have you looked at that yet? If so, does it make sense?