Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook