Find Moment Around A with 3D Vectors: B & C

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The discussion focuses on calculating the moment around point A using 3D vectors B and C, with a force magnitude of 1919N. The user attempts to find the correct r-vector for calculating the moment but is confused about which arm to use and whether their calculations are correct. They initially calculate the unit vector and force components but are advised to use the r-vector connecting point A to the force application point. The importance of using the correct r-vector and understanding the relationship between force and distance for calculating the moment is emphasized. The user seeks clarification on the proper approach to complete their homework assignment.
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Homework Statement


wbqf13.jpg


A (0,0,0)
B(5,6,1)
C(3,0,4)

|F|= 1919N

Homework Equations

The Attempt at a Solution



Cartesian:
B= {5i + 6j + 1k}
C= {3i + 4k}

rCB ={2i + 6j - 3k}

|rCB| = sqrt(22 + 62 + 32) = 7

Unit vector = rCB / |rCB| = {2/7i + 6/7j - 3/7k}

F = |F| * Unit vector = 1919 {2/7i + 6/7j - 3/7k} = 548.29i + 1644.86j - 822.43k

Now firstly, I'm not sure if I even did the above part correctly, but to find the moment I'm a bit lost and my don't seem to have much in my notes about this...

I think the next step is using cross product, but I'm a bit lost as to which arm to take to find moment about A...

If I use rBA I get:

MA = rBA x F = 6579.44i - 4660.44j - 4934.56k

WELP! Need help!

Cheers LB
 
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LeftBrain said:

Homework Statement


wbqf13.jpg


A (0,0,0)
B(5,6,1)
C(3,0,4)

|F|= 1919N

Homework Equations

The Attempt at a Solution



Cartesian:
B= {5i + 6j + 1k}
C= {3i + 4k}

rCB ={2i + 6j - 3k}

|rCB| = sqrt(22 + 62 + 32) = 7

Unit vector = rCB / |rCB| = {2/7i + 6/7j - 3/7k}

This is not the r-vector for use in calculating the moment about point A. What is the r-vector you need instead?

(Hint: What is the Force pulling on?)
F = |F| * Unit vector = 1919 {2/7i + 6/7j - 3/7k} = 548.29i + 1644.86j - 822.43k

Now firstly, I'm not sure if I even did the above part correctly, but to find the moment I'm a bit lost and my don't seem to have much in my notes about this...

You can always find discussions and examples of how to calculate the moment on the web.
I think the next step is using cross product, but I'm a bit lost as to which arm to take to find moment about A...

If I use rBA I get:

MA = rBA x F = 6579.44i - 4660.44j - 4934.56k

WELP! Need help!

Cheers LB
See above. Remember, the problem asks for the magnitude of the moment, so don't forget that!
 
So I need to use rBC instead?
 
LeftBrain said:
So I need to use rBC instead?
No, you need to use the r-vector which connects point A to the end of the rope pulling on the end of bar AB.

Remember, calculating a Moment requires a Force and some kind of Distance between the force and the axis of rotation caused by the moment.
 

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