Anyone good with vector problems involving moments?

- anonymous

Anyone good with vector problems involving moments?

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- jamiebookeater

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- anonymous

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

- anonymous

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

- anonymous

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.

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- anonymous

You're totally wrong.... sorry

- anonymous

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

- anonymous

@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).

##### 1 Attachment

- anonymous

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

- anonymous

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

- anonymous

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.

- anonymous

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

- anonymous

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?

- anonymous

Well noboyd would notice them if they stay sit.

- anonymous

Or maybe they had to go to the bathroom =P

- anonymous

HAHAHAHAHA I have something funny to mention

- anonymous

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.

- anonymous

Ohh poor girl! Five hours is insane!

- anonymous

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.

- anonymous

I wish you could help me with multivar calc. later

- anonymous

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

- anonymous

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

- anonymous

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

- anonymous

Which grade am I in? "Starting over!"

- anonymous

Sorry, but what were you studying before?

- anonymous

to get into pharmacy school

- anonymous

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

- anonymous

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

- anonymous

and now what is your major? Engineering?

- anonymous

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

- anonymous

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

- anonymous

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

- anonymous

It's okay.

- anonymous

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

- anonymous

It would crumble LOL!!!!!

- anonymous

Hahaha "structurally unsound"

- anonymous

I'm so used to that now.

- anonymous

Do you study there ATM?

- anonymous

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

- anonymous

I'll brb... sec

- anonymous

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!

- anonymous

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 :-)

- anonymous

what pdf?

- anonymous

Who is Jae?

- anonymous

My instructor.

- anonymous

Was your problem wrong?

- anonymous

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.

- anonymous

hahaha I know what a pdf is =P

- anonymous

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

- anonymous

I meant what was the content of the pdf?

- anonymous

ahhh!

- anonymous

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.

- anonymous

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

- anonymous

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

- anonymous

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.

- anonymous

Thank you. I'd apreciated that!

- anonymous

k, I'll create a link.

- anonymous

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

- anonymous

Perfecto!

- anonymous

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.

- anonymous

Maybe you're a different kind of learner.

- anonymous

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

- anonymous

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

- anonymous

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

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