Due date for this assignment is Friday 10/19 at 4 pm. These problems are all from Chapter 5 and give good practice on rigid body equilibrium problems. In particular, you get to see lots of different types of connections, so don’t forget to consult Tables 5-1 and 5-2 if you need guidance.
Probs. 5-26, 30, 32, 49, 53, 75, 83
As usual, use this post as an open thread for discussion.
19 responses so far ↓
1 kls2ycc // Oct 17, 2007 at 10:18 pm
Can any give advice on how to start Problem 32? I’ve tried solving the moment equation for the force at A, and tried using the Force equ. but I keep getting numbers that are about 3 off from the answers in the book. The only forces involved in the equilibrium equ. is the 100 force, force at A and force at B … correct?
2 kls2ycc // Oct 17, 2007 at 10:36 pm
oops… for the question before i meant problem 3o. thanks.
3 berger // Oct 18, 2007 at 12:45 pm
Well, yes, those are the only forces. But you also have the applied moment at the bend in the L, and isn’t there also something going on at A? Consult Table 5-1 if you are not sure about this.
4 tpadden // Oct 18, 2007 at 1:16 pm
On problem number 26, I am confused about how to go about finding the moment equilibrium equation. I guess I don’t really know which forces to include in the moment analysis and where you should do the moment from.
5 kls2ycc // Oct 18, 2007 at 2:55 pm
If you take the moment about B, then you have the 800 lbft moment and then the x component of the moment from the 390lb and an x and y moment component at A. I used this equ. to solve for the Force A. Since the y component in the 390 force is on the same axis as B is does not create a moment.
6 kls2ycc // Oct 18, 2007 at 3:04 pm
For problem 30, the only way I can think about starting this problem is solving the moment equ. first. Is this the right way to go about it? If so, do you start by finding the moment about B? I’ve tried setting the equation up several different ways, but I wasn’t sure whether the 200 lb*ft moment is included since its along the same axis? or if in this equ. i should solve for Ma of Fa?
7 jrh3 // Oct 18, 2007 at 4:23 pm
anybody have an idea for the numerical solution for number 32? it is the only question without something in the back of the book. thanks
8 digs // Oct 18, 2007 at 4:48 pm
For 5-30, I actually found the forces at A and B first, using Fx = 0 and Fy = 0 and the triangles they gave us. After that, I directly solved for Ma, using the force I found for B. It seemed a bit easier than trying to solve for the moment at B first. Hope that helps.
9 hambut // Oct 18, 2007 at 5:34 pm
digs, i definitely 30 the way you did, but i am getting numbers way off from the back of the book. you solved for one force, then substituted it into the other force equation and then solved for the moment correct?
10 Kate Villars // Oct 18, 2007 at 6:37 pm
I’m working on number 83 and I have 6 unknowns but only 5 equations: the sum of forces in x, sum of forces in y, sum of the forces in z, the moment about x, and the moment about y. I can’t figure out how to write one for the moment about z. Does anyone have any suggestions?
11 kls2ycc // Oct 18, 2007 at 7:12 pm
for 83, you need the moment about z = 0 if you solved it by use of vectors, the i, j, and k components go respectively with the moment x, y, and z
12 kls2ycc // Oct 18, 2007 at 7:30 pm
for 5-30 i tried to use the force equ the first couple of times, i also got numbers way off from the back of the book. for the Fx=0 there were only two forces correct the force at A and B, and then Fy=0 there were 3 forces, 100, force at A and at B? correct? or am I missing something
13 katie // Oct 18, 2007 at 7:41 pm
for 5-30, i tried to solve for forces at A and B too, but i kept getting 71 for the Force at B and 34 for the Force at A. are you doing something different to the triangles other than just using the numbers they gave you?
14 jrh3 // Oct 18, 2007 at 7:59 pm
kls2ycc, your force equations seem right. now just solve for Fa in terms of Fb (using the Fx=0 equation). take that Fb and put it in the Fy equation. once you solve for Fb, the others will fall into place. and watch out for those little triangles they give you. sometimes their direction is wrong and it messes up your sines and cosines
15 jrh3 // Oct 18, 2007 at 8:02 pm
and by wrong i mean their direction will point you in teh wrong direction. obviously they are not wrong be/c the book is infallible (for an example, see that problem 49 is presented in Newtons but the answer is presented in pounds. whoopsie there book)
16 tpadden // Oct 18, 2007 at 8:58 pm
how do you even go about staring problem 30, I’m so confused as to what equations to use?
17 tpadden // Oct 18, 2007 at 9:45 pm
and by 30 I really meant 32
18 rsa8q // Oct 19, 2007 at 1:36 am
does anyone want to check their answer for 32 with me? Also, any reason why the answer given in the back of the book for 49 is in lbs instead of Newtons?
19 berger // Oct 19, 2007 at 5:15 am
For 5-49, that’s just a typo…the numerical values are right but the units are wrong in the back of the book. Instead of kip, it should be kN.
For 5-32, T turns out to be a little over 9 lb. To start this problem, consider the different possible FBDs you can draw…like, what’s the easiest way to isolate the unknown tension?
Fur 5-83, make sure you use ALL the equilibrium equations (in 3D problems, there are 6 of them) to solve for the unknowns. Also make sure you model the connection to ground correctly–look at Table 5-2 if you need advice.
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