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Homework Help: Gravitational acceleration, cosine problem

  1. Nov 6, 2014 #1
    1. The problem statement, all variables and given/known data
    The gravitational acceleration at latitude x (0<x<90) can be estimated with g(x)=a*cos(2x)+b.
    1) Determine what a and b is if the gravitational acceleration is 9.780m/s^2 at x=0 and 9.832m/s^2 at x=90.

    2. Relevant equations

    3. The attempt at a solution
    So I begin by entering what I know:
    9.780=a+b (Because cos(2*0) is 1)
    And then the other equation:
    9.832=-5.9846*a+b (Because cos(2*90) is -5.9846)

    I then combine the two equations, jumping over the middle steps, gives:
    -0.00868=a-a which means... -0.00868=0 which isn't logical. I wanted to get what a is, then enter it into either of the equations and get b.
  2. jcsd
  3. Nov 6, 2014 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    cos(2*90) = -5.9846? cosine can only evaluate between -1 and +1. Remember, latitude is measured in degrees, not radians. Try again.
  4. Nov 6, 2014 #3


    Staff: Mentor

    You're skipping some steps here and writing stuff that isn't true. cos(2 * 90°) = -1.
  5. Nov 6, 2014 #4
    Thank you! The book said that real life applications are measured in radians and not degrees, so I assumed it would be the same here. I solved it now, thanks again!
  6. Nov 6, 2014 #5


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    Staff Emeritus
    Science Advisor
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    Your book is not correct on this point. Latitude and longitude are always measured in degrees. In most other applications, degrees or grads are used. Radians are generally used only in math or science, because they simplify working with derivatives and integrals of trig functions.
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