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Math Help - Uniform Accelerated Motion

  1. #1
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    Question Uniform Accelerated Motion

    Spotting a police car, you hit the breaks on your new porsche to reduce your speed from 90 mi/hr to 60 mi/hr at a constant rate over a distance of 200ft.
    a)Find the accerleration in ft/s^2
    b)How long does it take to reduce your speed to 55 mi/hr?
    c)At the acceleration obtained in part (a), how long would it take for you to bring your porsche to a complete stop from 90 mi/hr?

    I know this has something to do with these formulas
    s(t)=So+Vot+1/2gt^2
    v(t)=Vo+at
    but I am not sure how to apply them and where to start.
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  2. #2
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    I know that the answer is
    a) -121/5 ft/s^2
    b) 70/33 ft/s^2
    c)60/11s
    I just don't know how to get to this
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  3. #3
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    Quote Originally Posted by Kels14 View Post
    Spotting a police car, you hit the breaks on your new porsche to reduce your speed from 90 mi/hr to 60 mi/hr at a constant rate over a distance of 200ft.
    a)Find the accerleration in ft/s^2
    You missed a rather useful equation for when a is constant: \displaystyle v^2 = v_0^2 + 2a(s - s_0)

    But no matter. You know v_0 and v. You do not know t. v = v_0 + at. We need another equation with a and t in it. You also have \displaystyle s = s0 + v_0t + \frac{1}{2}at^2. <-- s_0 is 0 ft. Why?

    You now have two equations in two unknowns, a and t.

    Try that and we can get to the next two after you've got this.

    -Dan
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