# Homework Help: Pitch and wrench

1. Nov 8, 2008

### jaredmt

1. The problem statement, all variables and given/known data
please replace the force-couple system of the previous problem with a wrench. Please state the pitch of the wrench and the coordinates of the point at which the wrench axis intercepts the xy plane

2. Relevant equations

3. The attempt at a solution

ok there are 3 forces going in different directions at different points. the force-couple system they are referring too consists of the moment about point D (MD) and the resultant force (R) if I understand correctly.

MD = <2376.1, 950, -710>
R = <238.1, -46.9, 237.7>

note: 1 force created no moment because it was alligned with point D

pitch (p) = (R MD)/(magnitude of R^2) = 3.054

that much seems easy, im confident that i did that much correctly. I was trying what the book told me to do for the rest and I keep getting answers with infinity and unmatching answers. they said M1 = pR. so I did pR to get M1.
then they said: M1 + rXR = MD
so i need to figure out vector r. so i set rXR = MD - M1.
if i set r = <x,y,z> and plug that in then i get answers with infinity (for x,y, and z)
they ask for the xy axis so on my next attempt i set z = 0 and plugged in r = <x,y,0> then i got answers that didnt match...
any idea what I am doing wrong?

p.s.
i also tried just doing rXR = MD. i had the same results

OK i just relooked the part where i put in r = <x,y,0>
i came up with value for x and y but in the 3rd equation it doesnt add up to 0 like it should. it is off by .0722

is this a problem? or is it only a matter of signifigant figures? because if its just SF then i think i actually did the problem correctly. just let me know if you see anything wrong with the concept of what i explained

Last edited: Nov 8, 2008