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Change in Elevation-Trig.

  1. Dec 20, 2013 #1
    1. The problem statement, all variables and given/known data

    A 6.0 mile long section of railroad has an angle of inclination of 2.1 degrees. Find the change in elevation over the 6 miles. Give your answer in feet.

    2. Relevant equations

    3. The attempt at a solution

    Sin 2.1=6/hyp
    h=6 mi/2.1sin
    h=163.7 mi=864336 ft.

    Seems off, but I'm not sure. I think I did something backwards, maybe?

    Let's try this:
    h=2.1sin/6 mi
    h=0.006 miles.

    h=32.25 ft.

    This seems further off.
  2. jcsd
  3. Dec 20, 2013 #2
    The elevation gain would be the "opposite" side of the angle, not the hypotenuse.
  4. Dec 20, 2013 #3
    The opposite side should be the hypotenuse times the sine of the angle, not divided by the sine of the angle.
  5. Dec 20, 2013 #4
    Okay, thanks! I was trying to calculate the wrong angle.
    sine(2.1)=opp/320 m.
    Opp=11.7 m
    Opp=61776 ft.

    Like that? (-:
  6. Dec 20, 2013 #5
    That is how you would solve for a height, given a hypotenuse of 320m and angle of 2.1 degrees, yes.

    A few remarks. sin(2.1) should be a very small number, where did 0.37 come from? The conversion from meters to feet is WAY off, double check your work!
  7. Dec 20, 2013 #6
    Oh, I just misread the decimal place, and then for whatever reason my brain said "HEY! Let's multiply this number by 5280!"

    Opp=11.7 m

    Opp=38.5 ft.
  8. Dec 20, 2013 #7
    Much better! :)

    A good tip is to assess your answers after every problem; to see if they make sense. It helps in testing situations!
  9. Dec 20, 2013 #8
    Haha definitely. I usually do, but I'm multi-tasking. Which I will not be doing during my test. xD
  10. Dec 20, 2013 #9
    6 miles (hypotenuse) is about 30000 ft, and the sine of 2 degrees is about 0.03. So, 0.03 x 30000 is on the order of about 1000 ft.

    I have a comment about your multitasking and inattention to what you are doing. Those of us who are trying to help you are contributing our valuable time. It doesn't seem fair to us for you to inattentively multitask and require 5x as much assistance because of it.

  11. Dec 20, 2013 #10
    Sorry, I don't get the first bit. I was intending to convert at the end, are you just trying to show that we could have done it in the beginning? I think the angle I need to use sine for is 2.1 degrees. Did you think the answer was wrong, or are you just showing a different way to do it?

    I think we have different understandings of "multitasking." I was just thinking about several other things, I wasn't goofing off while posting the answer. It was a lighthearted joke. I'm not being "inattentive." I missed one part of the problem, and corrected it. It's kind of you and everyone else to try to help, but I don't require your assistance. I'm sure you have better things to do, so if you would rather do them, go for it! (-: Sorry for the misunderstanding!

    Thanks again for your help!
    Last edited: Dec 20, 2013
  12. Dec 20, 2013 #11
    I'm saying that the correct answer is about 1000 ft.
  13. Dec 20, 2013 #12

    There seems to be a confusion here. Could someone please confirm either answer?
  14. Dec 20, 2013 #13
    Sorry for the late reply, I was busy. The confusion is that I said your answer was correct for the third quote above, given that information (320m as the hypotenuse). Chestermiller is referring to your original post (2nd quote) which had 6 miles as the hypotenuse. The answers in the quotes aren't correct, but we've discussed that already.
  15. Dec 20, 2013 #14
    Oh, thank you! So how do I tell which is the hypotenuse if I can't see a right angle?
  16. Dec 20, 2013 #15
    Should I cut the triangle in half?
  17. Dec 20, 2013 #16
    The best answer to this is to draw a triangle yourself, if you haven't been doing it yet. It really helps to VIEW the problem instead of just trying to apply a formula.

    For these types of problems, when you draw the triangle you are viewing the problem from the side. Think of it as someone driving on a road and all of a sudden they start going uphill at a 2.1 degree angle. From this you can construct the triangle. A picture would help better explain it. The right angle is the corner of the triangle below:

    Attached Files:

  18. Dec 20, 2013 #17
    Okay, I see it now. Thank you! I usually try to draw diagrams, but I already had one. The angle of inclination was just so small that there seemed to be two right angles, even though that's not possible.
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