## brinethery 3 years ago Anyone good with vector problems involving moments?

1. brinethery

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

2. brinethery
3. eashmore

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.

4. No-data

You're totally wrong.... sorry

5. No-data

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

6. brinethery

@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).

7. No-data

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

8. brinethery

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

9. brinethery

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.

10. No-data

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

11. brinethery

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?

12. No-data

Well noboyd would notice them if they stay sit.

13. No-data

Or maybe they had to go to the bathroom =P

14. brinethery

HAHAHAHAHA I have something funny to mention

15. brinethery

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.

16. No-data

Ohh poor girl! Five hours is insane!

17. brinethery

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.

18. No-data

I wish you could help me with multivar calc. later

19. No-data

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

20. brinethery

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

21. brinethery

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

22. brinethery

Which grade am I in? "Starting over!"

23. No-data

Sorry, but what were you studying before?

24. brinethery

to get into pharmacy school

25. No-data

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

26. brinethery

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

27. No-data

and now what is your major? Engineering?

28. No-data

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

29. brinethery

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

30. No-data

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

31. brinethery

It's okay.

32. No-data

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

33. brinethery

It would crumble LOL!!!!!

34. brinethery

Hahaha "structurally unsound"

35. No-data

I'm so used to that now.

36. No-data

Do you study there ATM?

37. No-data

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

38. brinethery

I'll brb... sec

39. eashmore

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!

40. brinethery

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 :-)

41. No-data

what pdf?

42. No-data

Who is Jae?

43. brinethery

My instructor.

44. No-data

45. brinethery

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.

46. No-data

hahaha I know what a pdf is =P

47. brinethery

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

48. No-data

I meant what was the content of the pdf?

49. No-data

ahhh!

50. brinethery

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.

51. No-data

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

52. No-data

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

53. brinethery

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.

54. No-data

Thank you. I'd apreciated that!

55. brinethery

56. No-data

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

57. No-data

Perfecto!

58. brinethery

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.

59. No-data

Maybe you're a different kind of learner.

60. brinethery

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

61. eashmore

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

62. brinethery

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