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Homework Help: Mod of Sine Equation

  1. Jun 12, 2007 #1
    1. The problem statement, all variables and given/known data

    Guy records data of person using a swing. Comes up with an equation to show his findings: y = 1.6sin(1.8x - 0.9) + 2.5

    What does each number represent in relation to the situation (person swinging)?

    2. Relevant equations

    Don't think any equations are needed for this. But it's good to know that above equation is derived from y = asin(bx - c) + d

    a - vertical stretch/shrink
    b - horizontal stretch/shrink
    c - horizontal shift
    d - vertical shift

    3. The attempt at a solution

    This one's been killin me. Since the variable 'd' represents the vertical shift, I was guessing the 1.6 may represent the distance the person's off of the ground. And since 'a' is the vertical stretch/shrink, maybe that could represent the maximum height or something. I have no idea what 'b' and 'c' will represent.
  2. jcsd
  3. Jun 12, 2007 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    First, note that the sine wave oscillates between -1 and +1. So, 1.6 sin (...) oscillates between -1.6 and +1.6.

    If you take something that oscillates between -1.6 and +1.6, and add 2.5 to it, then it oscillates between -1.6 + 2.5 and 1.6 + 2.5.

    - Warren
  4. Jun 12, 2007 #3
    There is a relationship between b and the period of the function. Assuming this is a vertical position versus time function, the horizontal shift c is often called a phase shift and tells where equilibrium (y = 0 for a sine function) is in relation to the clock. With no phase shift, the function will be at a maximum at t = pi/2, a minimum at t = 3pi/3, etc.,
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