Can someone explain to me the signs on this vector problem

[pyroknife]

Homework Statement

For drawing purposes, I'm just going to state the locations of the 2 points. Cable AB carries a tension of 12 kN. Point A=(0,0,70) m and point B=(35,-25,10) m.
Find the corresponding force on point B as a vector.




okay now this part has me confused. I'm trying to find my r vector so I can get the unit vector out of that and multiply by the tension to get the corresponding force vector.

My question is do you do B-A or A-b to get the r vector? The only way I can match the signs in the book is to do A-B, but why would it be A-B instead of B-A?

 

[pyroknife]

can someone also answer the question why the E field in the conductor w/ the 2 radius is 0? Shouldn't there be an e field due to the smaller conductor?

 

[Dick]

The direction of the force on B has to point away from B. Tensions pull, they don't push. A-B points away from B. B-A doesn't.

 

[pyroknife]

oh thank you for explaining. r vector and force vector always point in same direction correct?

 

[Dick]

oh thank you for explaining. r vector and force vector always point in same direction correct?

I like my explanation better. Tension pulls. It should point AWAY from B.