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Homework Help: Physics Kinimatics/Energy Question

  1. Oct 7, 2009 #1
    If the speed of a car is increased by 40%, 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
    K=1/2 m v^2

    3. The attempt at a solution
    well i set kenitic energy to work so i got 1/2mv^2 = Fd, the force acting on the car is friction i then rewrote equation to V^2 / (2MG) = D, i got about 1.4 when i set velocity mass and gravity to 1. Didnt show up correctly on my webassign any suggestions?
  2. jcsd
  3. Oct 7, 2009 #2
    I would suggest that since you don't know velocity or mass initially that you don't arbitrarily assign them to values. However, what you DO know is by what factor it was increased from the initial value. Which means you can set up a ratio given that you know it takes distance D to stop for velocity V, you then set up your equation with your new velocity (1.4*V) and find the scaling factor in front of D. Those other values should all cancel out if you take it as a ratio.

    Hope this helps.
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