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Homework Help: Reasoning behind?

  1. Oct 26, 2008 #1
    1. The problem statement, all variables and given/known data

    If the speed of a car is increased by 50%, by what factor will its minimum braking distance be increased, assuming all else is the same? Ignore the driver's reaction time.

    2. Relevant equations


    .5(2m)(v1+5.0)^2 = .5m(v2+5.0)^2

    I know the answer is 2.25 and is deduced from the above equation. I tried to simplify:

    v1^2+10v1+25 = .5v2^2+5v2+12.5

    but I am still confused. I know I should eliminate time as a variable, but I am unsure of how to do this.
     
  2. jcsd
  3. Oct 26, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    I don't understand what this equation represents.

    Try this:

    KE_1 = 1/2 m (v_1)^2

    KE_2 = 1/2 m (v_2)^2

    If v_2 is 50% greater than v_1 (express that mathematically), how does KE_2 compare with KE_1?
     
  4. Oct 26, 2008 #3
    So,
    (v_1) = (v_2) + (v_2 * 0.5) = 1.5 (v_2)
    and, if I substitute KE_1 and KE_2 into the equation,
    I get KE_1=2.25KE_2.
    OMG THANK YOU!!!!!
     
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