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Homework Help: Grade 11 Kinematic Question.

  1. Nov 1, 2009 #1
    1. The problem statement, all variables and given/known data
    A car is traveling along a highway with a speed of 25m/s when the driver sees an obstruction 180m directly ahead. It takes the driver 0.80s to react and begin breaking.

    A) How long will it take the car to stop once brakes are applies, provided the car stops just before the obstruction?

    B) What is the value of acceleration of the car just before hitting the obstruction? Assume acceleration is uniform.

    2. Relevant equations

    d= v (t)

    a = v2-v1 / t2 - t1

    d = v1(t) + 1/2 a (t2)

    3. The attempt at a solution

    I don't understand how or know how to use two equations with missing variables to find a variable.
  2. jcsd
  3. Nov 1, 2009 #2
    Well the car is going at 25m/s whilst the driver is taking time to react, so you can work out the distance the car goes before the brakes are applied and take this distance away from 180m. This new number is the distance you need to work out the braking time for.

    Now you know the distance the car takes to brake, and the change in velocity (25m/s to 0m/s). Do you know a way to work out the acceleration required to stop the car?

    [tex]v^2 = u^2 + 2as[/tex]

    Where v = final velocity, u = initial velocity, a = acceleration and s = distance. Rearrange this equation to get acceleration.

    Once you have the acceleration (it should be a negative number, since velocity is decreasing with time), you can use your acceleration equation to work out the time taken. Remember to add on the 0.8s taken for the driver to react.
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