This is sort of a warm-up assignment dealing with vectors. There are multiple ways to solve these problems, and they all boil down to how you want to describe your vectors. You can use projections of vectors onto the coordinate axes (this is more of a scalar approach), or you can use Cartesian vectors, or you can use any other vector expression with which you feel comfortable. The important part of this assignment is to recognized that vectors–very important mathematical concepts–are also physically very relevant for this course.
From Chapter 2: 2-32, 2-39, 2-51, 2-75
Use this post as an open thread for discussion about this homework. Post freely and often; be open to sharing your wisdom.
Some Notes: 2-75 in particular will take some thought. The others you will probably be able to move through reasonably quickly. Also, remember that homework is not pledged, so please work with each other to develop your understanding of the material.
23 responses so far ↓
1 mdk7q // Sep 3, 2007 at 2:17 pm
when is this due?
2 jmwakey7145 // Sep 3, 2007 at 2:26 pm
its due friday. . . he mentioned it in class i believe
3 mdk7q // Sep 3, 2007 at 2:26 pm
i didnt hear him. thanks
4 berger // Sep 4, 2007 at 1:58 pm
Yeah, sorry to not be more clear. Due date is Friday at the beginning of class.
5 rsa8q // Sep 4, 2007 at 11:55 pm
if anyone would like to discuss/check answers, (esp. #51) email me at rsa8q or respond here. -bobby
6 John // Sep 5, 2007 at 12:12 am
For 2-51, I thought you would be able to simply find the combination of the forces from the two known forces and then that would equal the unknown force, just in the opposite direction and therefore the final resultant force would be zero and F would be the combined forces of the known forces. However, when I did this it is not the same answer as in the back of the book.
If you have any tips, I’d appreciate it.
7 rsa8q // Sep 5, 2007 at 10:53 am
John, thanks for the response. I tried that first as well, but then realized it was flawed because F and the resultant of the known forces are at different angles. I also tried a system of equations, but that was making it too complex. I got the answer using the geometry between the resultant of the known forces and F. Think of what geometry will make Fr the smallest.
8 Fox // Sep 5, 2007 at 10:06 pm
Problem 75….
Can anyone explain to me how to find the x,y, and z components of force vector 1….
The triangle with the 3,4,5 is throwing me off!
Thanks….
Mickie
9 Fox // Sep 5, 2007 at 10:08 pm
Just to make sure…
Don’t we have to find the components of all the known force vectors and subtract them from the resultant to find F3?
or…am I doing too much work?
Mickie
10 rsa8q // Sep 5, 2007 at 11:02 pm
mickie, your second comment is correct. then, if you use directional cosines you can find alpha, beta, and gamma.
about your first comment, try using similar triangles. (5 is to 80 as __ is to __)
11 Fox // Sep 6, 2007 at 9:40 am
I got the triangle thing..but what about the angles…
I do not understand how that picture makes any sense on finding the x,y,z components…because the plane runs along the x-axis not F1…so how is it relative to finding the angles for F1?
12 Vfort // Sep 6, 2007 at 5:08 pm
If you look at example 2.10 on pp 49, there’s an example of finding the cartesian coordinates of a vector using exactly the same information, just different values.
Hope that helps!
13 cmm8kb // Sep 6, 2007 at 6:46 pm
Anyone know if on 2-32 we should leave the angle as negative, or if it is positive because we are measuring “clockwise from the positive x-axis”?
14 cmm8kb // Sep 6, 2007 at 7:12 pm
Nevermind. I figure it’s positive.
15 cmm8kb // Sep 6, 2007 at 8:27 pm
In 2-51 can we assume that the 5kN force is perpendicular to the 4kN force? Anyone know?
16 cgo8a // Sep 6, 2007 at 9:21 pm
for 2-51 i think you can assume they are perpendicular. if you assume the 5kN force acts in the positive x direction and the 4 kN acts in the negative y you can solve for the magnitude of the resultant
17 sunset // Sep 6, 2007 at 11:22 pm
For 2-75…
im just confused, is the force F3 the same as the resultant force?
18 sunset // Sep 6, 2007 at 11:26 pm
whoops wrong diagram!
19 Fox // Sep 6, 2007 at 11:56 pm
yeah…I have worked on 75 probably for two hours…
I just cannot get the f1 components….
I understand how to find the answers….
It is very frustrating…
20 hambut // Sep 7, 2007 at 12:55 am
ok, so i am pretty sure i found the different components for the unknown force, since i double/triple checked it solving the resultant and whatev. but i am having serious problems trying to find the angles. i use the same format as the example in the book, but i cant get the same as the book. any advice por favor?
21 berger // Sep 7, 2007 at 7:29 am
Sounds like the problem on 2-75 is sort of a visualization, 3-D trig kind of a thing…would it be helpful for you to draw a series of 2-D projections of the problem (like, 3 2-D pictures) to help you visualize the details of the problem? I find this sometimes helps me see what is otherwise obscure.
PS I won’t be in today, so please turn in the HW some time before 5 pm (slide under my door).
22 kks5a // Sep 7, 2007 at 8:39 am
can anyone tell me where this door is that we’re supposed to slide our homework under? i can’t seem to find that anywhere. thanks.
23 nickd // Sep 7, 2007 at 9:20 am
309 MEC
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