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Homework Help: 1D Kinematics

  1. Jun 17, 2012 #1
    1. The problem statement, all variables and given/known data

    Investigating the scene of a car accident you measure the skid marks of one of the cars to be 100.0ft long. Assuming that the maximum deceleration on dry pavement is 0.75g, what was the minimum speed of the car(in mph) when it slammed on the brakes?

    2. Relevant equations

    x=x0 + v0t +1/2at^2
    v=v0 + at

    3. The attempt at a solution

    I know I need to solve for t, but I can't seem to wrap my ahead around setting up the equations for these types of problems. A helpful nudge would be appreciated.
  2. jcsd
  3. Jun 17, 2012 #2
    You don't need to solve for time, you are asked to solve initial velocity. You are given 3 values, displacement, acceleration, and final velocity.
  4. Jun 17, 2012 #3
    I was under the impression that most of these problems begin with solving for time. I'm really in the dark on this stuff. My instructor leaves much to be desired.
  5. Jun 18, 2012 #4
    Have you been taught all of the basic kinematic equations?

    If so, look at each of them. You'll find that each one has a different variable missing.

    Knowing this all you have to do is look for the equation with all the variables you're given and the variable you need and then use that.
  6. Jun 18, 2012 #5
    This is what I did...let me know if I'm on the right track.

    I used vf2 + v02 = 2as.

    so v02 = 2(7.35m/s2)(30.48m)
    v0 = 21.2m/s

    I think that's right, but I'm not sure. Then of course I have to convert to mph. Am I close?
  7. Jun 18, 2012 #6
    Yes, that is correct.
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