How Do You Calculate Work and Moment in Vector Problems?

[xcgirl]

Homework Statement

A = 2i - 3j + 3k
B = 4i +2j + k
C = i + 2j - 2k

a) Find the work done by force A to move an object C
b)Find the work done by the forces A and B acting together that moves an object C
c)Suppose point O is defined at the tail of C. Find the moment of force A applied at the head of C about O
d)Suppose point O is defined at the tail of B. Find the moment of force A applied at the head of B about O
e)Suppose point O is defined at the head of B. Find the moment of force A applied at the head of C about O

Homework Equations

W = Fdcos(theta)
|M| = d|F|
M = rxF

The Attempt at a Solution

I am pretty sure that I can do the moment problems but I am confused about finding the work since I've always learned that work is force multiplied by distance. In my book it said that work is force that acts in the same direction as the motion of a body, so I thought I would take the dot product.
I did A dot C and got 2i - 6J -3k

For the moment problems c and d I used M = rxF to get the answer. I think this is the right way to do it, but I don't know how to apply this method to part (e) because it involves A, B, and C

Can you tell me if any of this is the right thinking?

 

[radou]

For the moment problems c and d I used M = rxF to get the answer. I think this is the right way to do it, but I don't know how to apply this method to part (e) because it involves A, B, and C

Why do you think you can't apply "rxF" to (e)?