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Sinusoidal equations

  1. Mar 10, 2008 #1
    I have a horrible math teacher this year: she merely shows the steps to solving a problem and doesn't help us understand why and how it works.

    1. The problem statement, all variables and given/known data
    I need to find the equation for the height of a ferris wheel as it spins. It has a radius of 30m, and a center 18m above ground. It completes a cycle in 60s. The graph starts at the minimum

    I think I can write these when the x axis is pi/2, pi, 2pi, etc. But now it's in seconds.

    Max = 33m
    min=3m

    Amplitude is 15
    Period is 60s


    2. Relevant equations
    y=AcosB(X-C)+D



    3. The attempt at a solution
    A=15
    B= (cosine, so 2pi/60 or pi/30 ??????????)
    C= (cosine, starting at min, so moved left or right over by 1/2 period so 30???
    D= min is 3 meters above ground, so moved upwards period+3 , so 18??

    y=15cos((pi/30)(X+30))+18

    Now if my domain on my calculator is 0<x<60 and range 0<y<33 , then it looks good vertically, but it starts somewhere above 15 and not 3 ...not moved over enough.


    My math teacher is horrible and doesn't explain anything...please help!


    Edit: well that makes no sense. I graphed using an online calculator, and it worked fine. Then I tried on my calculator again and it was off. So I reset my calc. and tried again, and it worked. Is that still the correct answer ^ ?
     
    Last edited: Mar 10, 2008
  2. jcsd
  3. Mar 10, 2008 #2

    Dick

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    Science Advisor
    Homework Helper

    Teacher rage will get you nowhere. Sometimes you just have to live without them. Your answer y=15cos((pi/30)(X+30))+18 is already correct. Are you sure your calculator is set to use degrees instead of radians?
     
  4. Mar 11, 2008 #3
    It was all set fine - I later figured it must have been a bracket error. (the next few questions that I stumbled on were correct after I made sure the "2pi/period" value was enclosed fully...

    (But really: I actually have talked to about 10 people who have had her and share similar concerns. She's extremely nice, but her teaching style is...."weak" when compared to past Math teachers I've had and others I've heard of. I have her in 2 subjects - she's weak at helping us understand and is more about telling us how to memorize the steps. I don't know why I'm doing something, just how it's done. It's not the typical "I'm 17 and hate school derrrrrr" attitude - she is actually not that great of a teacher in certain areas.)
     
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