Question

Anyone good with vector problems involving moments?

Answer 1

Is my method correct or was I reading the problem wrong?

Answer 2

http://www.scribd.com/doc/86313887/Problem-6

Answer 3

I don't understand what they mean by "equally inclined to the positive ends of the orthogonal axis," but your methodology looks correct and complete.

Answer 4

You're totally wrong.... sorry

Answer 5

Just kidding. Your method is correct and you get the right answer. Congratulations.

Answer 6

@eashmore my instructor writes all of these problems and this was the practice final exam. I think he wanted to make the questions as hard as possible. He wanted us to work backwards (r crossed with F = moment, so find the components of F).

Equally inclined means this: You have an angle with the x-axis, with the y-axis and the z-axis. (Sorry if this info is nothing new to you!). A vector in 3-space has those 3 direction angles: alpha, beta, and gamma. If all angles are equally inclined, you can say:

alpha=beta=gamma = theta (we'll just use theta so as not to cause confusion).

If you square the cosine of each angle, that will equal 1.

cos^2(alpha) +cos^2(beta) +cos^2(gamma) = 1

Since the angles equal eachother (equally inclined with the positive axes, we can say:

cos^2(theta) +cos^2(theta) +cos^2(theta) = 1

3 cos^2(theta) =1

cos^2(theta) = 1/3

cos (theta) = 1/sqrt(3)

Therefore, all alpha, beta, and gamma have direction cosines of 1/sqrt(3)

So after all of that work, your unit vector will be:

(1/sqrt(3))i +(1/sqrt(3))j +(1/sqrt(3))k

Pg. 29 seems to explain this concept fairly well (see attached).

Answer 7

I can give you harder problems if you want to practice.

Answer 8

Well it was just the wording. Sometimes I have to read the question a few times to figure out what he wants.

Answer 9

To be fair, some other people were on here last night and had no clue what was going on. Well I finished my final today and I'm all "vectored" out.

Answer 10

Yeah that equally inclined thing it's weird. Haha I bet you'll get a good grade.

Answer 11

I will tell you what I don't get. The instructor gave 2 hours for the test. Some people finished in 50min, got up and left. Why not just sit around for the whole 2 hrs?

Answer 12

Well noboyd would notice them if they stay sit.

Answer 13

Or maybe they had to go to the bathroom =P

Answer 14

HAHAHAHAHA I have something funny to mention

Answer 15

I took multivar. calc last quarter and my teacher had us take the exams in the testing center. My classmate would sit in the testing center for like 5 hours taking her test. I asked her how she resisted going to the bathroom. She's like... I just hold it in.

Answer 16

Ohh poor girl! Five hours is insane!

Answer 17

My instructor was really tough. The exams were all application-based and it had been 5 years since I'd been into math. My dad convinced me to sign up lol.

Answer 18

I wish you could help me with multivar calc. later

Answer 19

I don't quite understand. In which grade are you?

Answer 20

I changed my mind about what field I want to go into.

Answer 21

I took 3 quarters of calc starting when I was 17 and I'm 24 now.

Answer 22

Which grade am I in? "Starting over!"

Answer 23

Sorry, but what were you studying before?

Answer 24

to get into pharmacy school

Answer 25

Did you want to be a mathematiciand and then changed your mind?

Answer 26

Nope, I am retarded at math compared to other people.

Answer 27

and now what is your major? Engineering?

Answer 28

I know you're not retarded but I can say is useful to think that haha.

Answer 29

I think civil would be a good path. And hopefully I could get a job with county... anything having to do with infrastructure.

Answer 30

Ahh yeah! I remember you told me that long time ago!

Answer 31

It's okay.

Answer 32

If I had to build a house I would call you =P

Answer 33

It would crumble LOL!!!!!

Answer 34

Hahaha "structurally unsound"

Answer 35

I'm so used to that now.

Answer 36

Do you study there ATM?

Answer 37

Please tell me when you get into statics.
I have plenty of doubts on that!

Answer 38

I'll brb... sec

Answer 39

Thanks for the explaination of 'equally inclined.' @brinethery I've never heard that expression.

Civil is a great field if you're interested in infrastructure. Everything from sky-scrapers to drainage ditches!

Keep up the great work! You're problems are always challenging and then you end up solving them before I can!

Answer 40

My teacher responded to my email this morning after I sent him the pdf file of what I wrote. He's like... no, no! Here's how you do it! Again, Jae saves the day :-)

Answer 41

what pdf?

Answer 42

Who is Jae?

Answer 43

My instructor.

Answer 44

Was your problem wrong?

Answer 45

You know the program Adobe that opens up those document files where you can only view them and do a tiny bit of editing? That's a pdf file.

Answer 46

hahaha I know what a pdf is =P

Answer 47

The original problem was wrong. But then I uploaded the revision to Scribd. The link remains the same if it's a revision.

Answer 48

I meant what was the content of the pdf?

Answer 49

ahhh!

Answer 50

I think I trashed the original, but it's wrong lol. But the current version is correct.

I thought he was saying that F sub x and F sub y were equally inclined. But he corrected me on that.

Answer 51

That is great! I wish I had teachers like him =P

Answer 52

I had one but I failed miserably in his class. Shame on me.

Answer 53

Actually he's teaching engr 106, but another instructor writes all of the exams and final exam. So that's probably why the exams are worded so strangely.

Answer 54

Thank you. I'd apreciated that!

Answer 55

k, I'll create a link.

Answer 56

I want so bad to understand Maxwell's equations, but I suck at Calculus hehe

Answer 57

Perfecto!

Answer 58

Sucks, I wish I had all the solns for at least one textbook. I don't think of it as cheating if I'm reading solutions as though I would read a textbook. It just helps to better understand things.

Answer 59

Maybe you're a different kind of learner.

Answer 60

Sorry @eashmore we can bring the convo outside of here if you keep getting pop-up messages.

Answer 61

Doesn't bother me. The wealth of notifications strokes my ego a bit. :-)

Answer 62

Hahaha I'm totally abusing the system :-). I'll try to bring it away from here.