# Question about direction of Moments 3d

baird.lindsay

## Homework Statement

knowing the tension in cable AB is 570 N, determine the moment about each of the coordinate axes of the force exerted on the plate at B . This link has a picture http://www.chegg.com/homework-help/...oordinate-axes-force-exerted-plate-b-q3031608

My question is when I am doing the cross product I do r X F . so <900, 0,0> X <450, -350, -180>

Instead of Fsub AB I do F sub BA so < 900,0,0> X < -450, 350,180> and I get an answer the opposite sign. Does it matter which way I find the force vector? and are both answer equivalent?

162j-270K or -162j +270K

I know how to solve the problem I just dont understand why you can use the tension from AB or BA because the answer will have different signs.

r X F

## The Attempt at a Solution

Staff Emeritus
Homework Helper
Yes, it does make a difference.

The force vector in your first cross product is incorrect, but it is correct in the second cross product.
The correct force vector for the tension in AB must have a negative x-component.

baird.lindsay
Yes, it does make a difference.

The force vector in your first cross product is incorrect, but it is correct in the second cross product.
The correct force vector for the tension in AB must have a negative x-component.

Why does it have to have a negative x component?

Unit Vector BA =< -450, 300, 180>
Unit Vector AB=< 450, -300, -180>

My solution manual is using BA but my teacher is saying it doesn't matter so I am confused. Please explain.. Thanks in advance:)

Staff Emeritus