Due date is Friday Sept. 21 at 4 pm. These problems are all from Chapter 3 and will reinforce your understanding of vectors in addition to challenging your understanding of particle equilibrium.
Problems: 3-10, 3-19, 3-33, 3-36, 3-45, 3-48, 3-56
- Prob. 3-39 is not assigned for homework, but is nonetheless a very nice problem which would be well worth your time to consider.
- Prob. 3-36 is quite a bit like the problem we did in class (and some students did for extra credit).
- All the 3-D problems resemble the problem we did in class on Friday Sept. 14.
As usual, use this post as an open thread for discussion…
14 responses so far ↓
1 dtr5a // Sep 19, 2007 at 1:26 am
I’m having trouble setting up the equilibrium equations for problem #19 and was wondering if anyone had any suggestions. I’m confused because it seems as though point B exerts a negative force by pulling down on C, but also an upward force by pulling up on A and I’m not sure how to factor that in or even if you have to…
2 berger // Sep 19, 2007 at 12:08 pm
I wanted to chime in on something…on this HW assignment you may find some numerical tools valuable. For instance, you might wish to do some matrix algebra on your calculator for one of the problems. Or, you might have some trig to do and it is easiest to just use a numerical tool like MathCAD.
3 Laura Ostanek // Sep 19, 2007 at 1:12 pm
dtr5a, because CA and CB are part of the same rope, they have the same tension. You can consider them to be two separate forces acting on the point C with the same magnitude but different directions. Hope that helps.
4 jmwakey7145 // Sep 20, 2007 at 7:35 am
i am still confused about how to set up the force equations for problem 3-19. If anyone could help that would be great
5 berger // Sep 20, 2007 at 11:46 am
Laura is right on 3-19…draw an FBD @ C and you will end up with two unknowns: the tension CD and the angle θ. Then you can use the sum of forces in x and y to develop the equations you need.
6 jmwakey7145 // Sep 20, 2007 at 1:53 pm
isn’t the weight of the crate also an unknown?
7 John // Sep 20, 2007 at 3:40 pm
For 3-33, how do you find the vertical height of the two wires?
8 berger // Sep 20, 2007 at 4:11 pm
Any takers on John’s question? I’ve had conversations with some of you regarding this, and my not-so-well-formed comment above has some clues…when all else fails, start writing some equations (trig, in this case) and see what you get…
9 DanielJ // Sep 20, 2007 at 6:14 pm
John, not sure this is right, but I used the law of cosines.
10 Alessandro // Sep 20, 2007 at 6:33 pm
That is right. You need to begin constructing triangles. There is one triangle where you will be able to immediately calculate its hypotenus using a^2+b^2=c^2. From there, you can simply use the law of cosines to find the angles you need.
11 John // Sep 20, 2007 at 7:34 pm
Thanks for the hints, in fact the law of cosines is not even needed. I used a bunch of pythagorean-theorem equations and got the correct answer for 3-33.
12 DanielJ // Sep 20, 2007 at 8:16 pm
Is there any way we can get the answer for number 3-56 posted so we can see if we got it right?
13 rsa8q // Sep 20, 2007 at 10:18 pm
if anyone would like to check the answers not in the back of the book, let me know
14 berger // Sep 21, 2007 at 8:47 am
Looks like for 3-56, the largest weight is 25.7 kN, which means the largest mass is about 2621 kg.
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