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Math Help - directly proportional

  1. #1
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    directly proportional

    Please help me show how to explain this problem to many kids of different backgrounds... I know the answer is 312.5...

    The distance required for a car to stop is directly proportional to the square of its velocity. If a car can stop in 112.5 meters at 15 km per hour, how many meters are needed to stop at 25 kilometers per hour?
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  2. #2
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    e^(i*pi)'s Avatar
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    d_1 = kv_1^2 where k is a non-zero constant.

    d_2 = kv_2^2


    If you divide the first equation into the second one you can eliminate k

    \dfrac{d_2}{d_1} = \dfrac{kv_2^2}{kv_1^2} = \dfrac{v_2^2}{v_1^2}


    Solve for d_2 since everything else is given
    Last edited by e^(i*pi); March 21st 2011 at 10:55 AM. Reason: specifying k is non-zero so it can cancel later
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