I hate to do this to ATOT, but...

[makken]

I hate to do this to you guys, but I can't seem to figure this one out:

"Determine the magnitude of moment of force P about line OC"

P is 50N directed from point A (0,8,9) to point B (0,0,15)
line OC is from the origin (0,0,0) to point C (12,8,9)

All dimensions are in meters.

The given answer is 424Nm, but I can't seem to figure out how they worked that out.

any pointers?

 

[Zim Hosein]

Originally posted by: makken
I hate to do this to you guys, but I can't seem to figure this one out:

"Determine the magnitude of moment of force P about line OC"

P is 50N directed from point A (0,8,9) to point B (0,0,15)
line OC is from the origin (0,0,0) to point C (12,8,9)

All dimensions are in meters.

The given answer is 424Nm, but I can't seem to figure out how they worked that out.

any pointers?

Yes, but you won't like 'em makken! 😛

 

[JJChicken]

Courtesy Bump

 

[SsupernovaE]

I only answer these questions in forums that support latex because I can do the math in Scientific Word and export latex to the forum. I do this because I use speech recognition to input data into the computer and inputting special math characters into this browser is physically exhausting. Sorry bro.

Paging Dr. Pizza!

 

[Brigandier]

I won't do it all, but use points A and B to figure out the direction of the force. Then cross the distance from the force to the line with the force and you have the moment.

 

[Fenixgoon]

since you're taking the moment, it should be the cross product of the force vector with your direction vector.

alternatively, you can do the dot product to find the angle between the two, then use that to get the component of the force that is perpendicular to the line and find the moment that way.

 

[Howard]

Whoa, nostalgic. Not.

 

[zzuupp]

Now that you've got two good pointers, get back to work

 

[makken]

Thanks for the pointers, here's what I have so far:

P is 50N along -8j + 6k, which is 50N * (-8j + 6k)/|M(ab)| = 50N *(-8j + 6k)/10 which is
-40j + 30k

M(oc) = M(o) *dot* u(oc) where M(o) = r(oa) X P

using r(oa) = 0i + 8j + 9k, r(oa) X P =
= -120i

u(oc) = (12i + 8j + 9k)/17

so M(o) *dot* u(oc) = -84.7Nm or 84.7Nm clockwise
... I can't seem to figure out where i'm going wrong