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TrojanPoem

  • one year ago

Mechanics

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  1. TrojanPoem
    • one year ago
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    @phi

  2. Jack1
    • one year ago
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    what specifically about mechanics? do u have an engineering based question?

  3. TrojanPoem
    • one year ago
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    Nah, general question.

  4. TrojanPoem
    • one year ago
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    Are you an engineer ?

  5. phi
    • one year ago
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    |dw:1444139335225:dw|

  6. Jack1
    • one year ago
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    nah, just ok at maths

  7. phi
    • one year ago
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    if we use the Law of Cosines on that triangle |dw:1444139454924:dw|

  8. TrojanPoem
    • one year ago
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    That's why I was getting the wrong angles -,-

  9. phi
    • one year ago
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    x is the angle between u and v but in the triangle the "opposite angle" is 180 - x cos (180-x) = - cos(x)

  10. phi
    • one year ago
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    a picture drawn to scale is very helpful. (If done very accurately, you can measure the answer), but even a rough drawing gives a good estimate of the answer (i.e. angle and length of the vector)

  11. TrojanPoem
    • one year ago
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    How ? It wasn't clear 131 ? on the last question

  12. phi
    • one year ago
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    Using the Law of Cosines idea? |dw:1444139889064:dw|

  13. TrojanPoem
    • one year ago
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    |dw:1444139666557:dw|

  14. TrojanPoem
    • one year ago
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    Why isn't 180 - Alpha = alpha ? Symmetry

  15. phi
    • one year ago
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    alpha (apparently, based on the answer choices) is the angle between v and f the angle between u and v is alpha + theta

  16. TrojanPoem
    • one year ago
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    |dw:1444140291240:dw|

  17. phi
    • one year ago
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    yes. with theta= 40, you get 120-alpha now it will work

  18. phi
    • one year ago
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    *140 - alpha

  19. TrojanPoem
    • one year ago
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    300/ sin alpha = 256.9 / sin40 sinalpha^-1 = 48.64 -,-

  20. TrojanPoem
    • one year ago
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    What am I doing wrong ?

  21. phi
    • one year ago
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    I think it would help if you drew a picture that more closely matched the situation

  22. TrojanPoem
    • one year ago
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    |dw:1444140790074:dw| That's our start then |dw:1444140826725:dw| then using the sine rule.

  23. phi
    • one year ago
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    as you know the Law of Sines is ambiguous when you have an obtuse angle notice sin(131.4)= sin(48.6) In this problem, you have to recognize you should choose the 131.4

  24. phi
    • one year ago
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    |dw:1444141166027:dw|

  25. phi
    • one year ago
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    V almost doubles back on u, and we have an obtuse angle opposite the longest side = 300 Considering these subtleties, perhaps using components (of vectors) is more straightforward

  26. TrojanPoem
    • one year ago
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    Using vectors'd give me the answer directly, however, I've to answer this question using cosine / sine rule :/

  27. phi
    • one year ago
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    the other way around the ambiguity is choose the acute angle rather than alpha

  28. TrojanPoem
    • one year ago
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    140 - alpha ?

  29. TrojanPoem
    • one year ago
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    Alpha doesn't seem like an obtuse in the drawing :/

  30. phi
    • one year ago
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    |dw:1444141658047:dw|

  31. TrojanPoem
    • one year ago
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    Fine, thanks phi :)

  32. phi
    • one year ago
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    \[ \frac{\sin (140-\alpha)}{60}= \frac{\sin(40)}{256.9}\]

  33. TrojanPoem
    • one year ago
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    Phi, I noticed that 256.9 is out of the choices -,-

  34. TrojanPoem
    • one year ago
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    a)112.7 b ) 145.9 c) 154.3 d ) 257.9

  35. TrojanPoem
    • one year ago
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    @phi

  36. phi
    • one year ago
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    Yes, I noticed that before. I assume it is a typo. Here are the 3 vectors

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  37. TrojanPoem
    • one year ago
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    Ok, as you're sure.

  38. phi
    • one year ago
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    You can ask your teacher, but the numbers work out to 256.9

  39. TrojanPoem
    • one year ago
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    Yeah, 257.9 works too with 131.4

  40. TrojanPoem
    • one year ago
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    u+v= f so v= f-u f=<300,0> (we let f point along the x-axis u= 60<cos 40, -sin 40> v= <60 cos(40) - 300, -60 sin(40) > now find the magnitude and angle of v |v| = 256.9 angle = atan(y/x) = 8.6 deg in the 3rd quadrant= 171.4 alpha = 171.4-40= 131.4 deg maybe it's a typo we've two solutions here which come with the same result.

  41. phi
    • one year ago
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    Yes, I'm sure the magnitude is 256.9 But complain to your teacher.

  42. TrojanPoem
    • one year ago
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    I will.

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