help me with my math (VECTORS ahh!)

[dannyho]

For some reason I'm having areally hard time with this statics problem. It is related to the car, and will help me make a useful spreadsheet (which I will post here) But I just can't get it to work, too much time away from the text book i guess.
So I've broken it down insto a statics problem where sum of moments =0 and sum of F=0.
I'm trying to do this without any numbers so that values can be easily changed, maybe that's why its giving me trouble

Force .......................Point acted at
-(F1)a j +(F1) k ........(0, b, 0)
-(F2)a j +(F2) k ........(0, 0, 0)
-(F3)a j +(F3) k ........(c, b, 0)
-(F4)a j +(F4) k ........(c, 0, 0)
aW j - W k ...............(d, 0.5b, e)

Trying to Solve for F1, F2, F3, and F4, in terms of (the givens) a, b, c, d, e and W)

Thought it might be a long shot posting it here, but it couldn't hurt.

Thanks
Dan

[Fuelie Fan]

I haven't finished going through it yet, but already it's clear that one of your forces will come out negative or else they won't sum to zero...was that what you mean by it not coming out right?

[Fuelie Fan]

my 5 equations (sum of forces in x is trivial);

-F1-F2-F3-F4+W = 0
F1+F2+F3+F4+W = 0
F1b+F3b+.5bW-acW = 0
-F3c-F4c-dW = 0
-F3ac-F4ac+daW = 0

[dannyho]

your second equation can't be right, I think you had meant to write -W
the problem is when i go to solve those questions i just keep running around in circles and never separate the 4 F's

[Fuelie Fan]

Tried solving them using matrices?

[dannyho]

I guess that's next, just keep moving to tecniques i hate more and more. Oh and It was my mistake with the problem, the last force was aW j - Wk, the first post has been fixed accordingly

[BADVELLE]

What the ......, go ahead and tell me to ! Sorry, looks like a bunch of numbers and a few letters to me!

[Fuelie Fan]

hmm i can't get an inverse using excel, so something must still be wrong with the problem. We've got more equations than unknowns, which isn't necessarily a bad thing (better than not having enough equations right?), but i'm still not positive that you've defined the forces correctly.

[dannyho]

Fuelie, this is what I'm getting for equations
sum Fx =trivial
sum Fy = 0 = -F1-F2-F3-F4+W
sum Fz = 0 = F1+F2+F3+F4-W (redundant)
sum Mx = 0 = (F1 +F3)b - 0.5Wb - eaW
sum My = 0 = dW - F3c - F4c
sum Mz = 0 = DaW -aF3c -aF4c (reduntant)

Leaving me 3 equations and 4 unknowns. There's no information missing, why does this hurt so much

[trapin]

Here Dan...let me give this a shot.

-F1-F2-F3-F4+W =
F1+F2+F3+F4+W =

F1b+F3b+.5bW-acW =
-F3c-F4c-dW =

-F3ac-F4ac+daW =

Hope it helps.

[dannyho]

well i don't know what went wrong but I endedu p finding a really good spreadsheet somewhere on the net. should help a lot

[Fuelie Fan]

I think the problem could be with not breaking the forces up into their components correctly? You don't have all of W in both j and k directions, for example. You may need to write the trig out as well.

[6'9"Witha69]

Here Dan...let me give this a shot.

-F1-F2-F3-F4+W =
F1+F2+F3+F4+W =

F1b+F3b+.5bW-acW =
-F3c-F4c-dW =

-F3ac-F4ac+daW =

Hope it helps.

I forgot most of this stuff after college.

[BMF Machine]

Here Dan...let me give this a shot.

-F1-F2-F3-F4+W =
F1+F2+F3+F4+W =

F1b+F3b+.5bW-acW =
-F3c-F4c-dW =

-F3ac-F4ac+daW =

Hope it helps.

I am rolling, thanks for the laugh Tony! I needed it!