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Qualification => University => Topic started by: SGVaibhav on June 14, 2010, 06:07:44 pm

Title: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: SGVaibhav on June 14, 2010, 06:07:44 pm: Resolved :D

Need help out here, mostly in finding moment about an axis.

1. The problem statement, all variables and given/known data
The member is supported by a pin at A and a cable BC. If the load at D is 300, determine the x, y, z components of reaction at these supports.
i.e Find the reactions at A and the tension in the cable BC

2. Relevant equations
Since it is in equilbrium.
$\Sigma F = 0$
$\Sigma M = 0$

3. The attempt at a solution
The most confusing part for me, is moment about an axis. (If possible, please tell me, how to find moment about an axis - the direct method and through the vector matrix).

Correct me, if i am wrong.

rBC = 0.3i - 0.6j + 0.2k
|rBC| = 0.7
uBC = 1/0.7 (0.3 i - 0.6j + 0.2k)

TBC = |TBC|/0.7 (0.3i - 0.6j + 0.2k)

Now
$\Sigma Fz = 0$
$= -300 + Az + TBC(0.2 / 0.7)$
Az is unknown, so to determine Az - we find moment about the x axis of B.
This will give, Az = 0
$\Sigma Fz = 0$
$= -300 + TBC(0.2/0.7) = 0$
TBC = 1050 N.

$\Sigma Fx = 0$
Ax + 1050(0.3/0.7) = 0
Ax = -450 N

$\Sigma Fy = 0$
Ay - 1050(0.6/0.7) = 0
Ay = 900N

Thats what i could do max (i think im right), now only moment about Y-axis w.r.t a and moment about Z-axis is left, w.r.t a again. (May and Maz).
Please tell me how to get this part, with 2-3 methods to get moment about an axis (Direct and through matrix).

Thanks a lot =)
Title: Re: 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: elemis on June 14, 2010, 06:09:42 pm: May God help you with this question, because I certainly cant :P
Title: Re: 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: SGVaibhav on June 14, 2010, 06:27:45 pm: Quote from: Ari Ben Canaan on June 14, 2010, 06:09:42 pm
May God help you with this question, because I certainly cant :P
i just notified the topic.
if u see astar, plz tell him to solve the moments part with explanation.
im off for now..
Title: Re: 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: elemis on June 15, 2010, 06:58:46 am: Send him a PM, if you havent already :)
Title: Re: 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: SGVaibhav on June 15, 2010, 09:12:11 am: sent..... but he has not come online from long time.
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: SGVaibhav on June 15, 2010, 09:39:45 am: i got it.....
but where is astar ???
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: elemis on June 15, 2010, 09:45:53 am: He was online a few hours ago.
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: astarmathsandphysics on June 15, 2010, 10:38:01 am: I am here now. Am looking at question
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: astarmathsandphysics on June 15, 2010, 11:00:18 am: moments about A for tension in BC
0.6j*-300k=Tx(-0.1i+0.2k)
Tx(-0.1i+0.2k)=-180i
T=-180i.180i/((-0.1i+0.2k).(-180i)) =-32400/-36j=-900j
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: astarmathsandphysics on June 15, 2010, 11:02:47 am: The diagram is difficult to make out. is BC paralell to the y axis?
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: SGVaibhav on June 15, 2010, 11:35:00 am: erm no...

rBC = 0.3i - 0.6j + 0.2k
|rBC| = 0.7
$uBC = (0.3i - 0.6j + 0.2k) / 0.7<br />Tbc = Tbc * (0.3i - 0.6j + 0.2k) / (0.7)<br />$
Since there is no reaction Max (M_Ax)

$\Sigma Mx = 0$
(-300 x 0.6) + (0.2 / 0.7 x Tbc) = 0
Therefore Tbc = 1050 N
I did not understand the part in bold. Why is only the k component considered in this equation?
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: astarmathsandphysics on June 15, 2010, 11:36:54 am: I cant make out the diagram properly. Is the bar holding the weight at z=0
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: SGVaibhav on June 15, 2010, 11:39:18 am: Quote from: astarmathsandphysics on June 15, 2010, 11:36:54 am
I cant make out the diagram properly. Is the bar holding the weight at z=0
yep =)
see my calculations
its all correct, but there is one part in the bold, which i dont understand.
lol i edited it like 5 times to put sigma properly
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: astarmathsandphysics on June 15, 2010, 11:49:53 am: is BC paralell with y axis?
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: SGVaibhav on June 15, 2010, 12:12:50 pm: Quote from: astarmathsandphysics on June 15, 2010, 11:49:53 am
is BC paralell with y axis?
nop
the position vector rBC describes it path from B to C.
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: astarmathsandphysics on June 15, 2010, 12:26:34 pm: I cant understand the question. cIf you get it from somewhere can you scane it in or post a link?
Title: Re: [Solved] 3 Dimensional Equlibirum - Statics (Astar, plz help)
Post by: SGVaibhav on June 15, 2010, 12:39:36 pm: oh shoot
i found this question in google ebook

dont mind the units..