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Homework Help: AP Physics mechanics problem

  1. Sep 5, 2003 #1
    I'm doing an independent study in AP Physics, and am having a lot of trouble with the following problem:

    "To stop a car, first you require a certain reaction time to begin braking; then the car slows under the constant braking deceleration. Suppose that the total distance moved by your car during these two phases is 56.7 m when its initial speed is 80.5 km/h, and 24.4 m when its initial speed is 48.3 km/h. What are (a) your reaction time and (b) the magnitude of the deceleration" (Fundamentals of Physics 6th ed; Halliday, Resnick, and Walker; pg 29, #37)

    I've set the problem up, graphed the data, manipulated a lot of equations, and tried to solve it geometrically. I suspect that I need to use ratios to relate the two different situations, solve for reaction time, and use that to find acceleration, but I can't see how to do it.

    Please Help!
  2. jcsd
  3. Sep 5, 2003 #2


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    Science Advisor

    Greetings !

    Use the formula: V^2 = V0^2 +/- 2 * a * x
    V - final velocity
    V0 - enitial velocity
    -a/+a - de/acceleration
    x - distance

    TO USE THE ABOVE FORMULA (with some small changes) AND
    FAILED ! :wink:

    Write the formula for each case. This is how it'll look :

    1: V1^2 = V01^2 - 2 * a * (X1 - t * V01)
    2: V2^2 = V02^2 - 2 * a * (X2 - t * V02)

    X1/X2 - given distances
    V01/V02 - given enitial velocities
    t - reaction time (unknown)
    a - acceleration (unknown)
    V1/V2 - final velocities per case => V1 = V2 = 0

    Live long and prosper.
  4. Sep 6, 2003 #3
    Thanks! You've put my troubled mind at ease.
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