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Math Help - Free fall

  1. #1
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    Post Free fall

    Equation given -- s=6.1t^2, t is seconds.
    Object dropped from 75 feet. Find the speed of the object at t = 3.

    one question- is the 75 feet important? Or do you just plug in the 3 for t to get 54.9 ft/sec?

    Any help is appreciated!
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  2. #2
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    In the problem speed of the object is required.
    Speed v = ds/dt.
    In the problem 75 feet is necessary. Otherwise the object may reach the ground before 3 seconds.
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  3. #3
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    ok..then how would I set this problem up?
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  4. #4
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    They are a lot of things wrong with the original post. First, let's look at the vertical motion equations on Earth:
    s(t) = -16t^2 + v_0 t + s_0
    v(t) = -32t + v_0

    t = time (I'll assume it's in seconds)
    s_0 = initial height
    v_0 = initial velocity
    s(t) = height after t sec
    v(t) = velocity after t sec

    The equation you've given us is
    s = 6.1t^2
    Does up => positive and down => negative? Or the other way around? If it is the former, then this equation is for an object that floats upwards. (In other words, is there a negative sign missing?)

    Since the object is being dropped, the initial velocity is zero, so it's okay that there is no t term in the equation you gave us. But there is mention of the initial height at all. So the question is, is this the equation you meant to give us?
    s(t) = -6.1t^2 + 75

    Or do you just plug in the 3 for t to get 54.9 ft/sec?
    Nope. The equation above looks like a position equation, not a velocity equation. Using the equation as originally given, plugging 3 in for t gives us a height of 54.9 feet, NOT a velocity of 54.9 ft/sec.


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