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Simple Question, Wording Unsure

  1. Jul 20, 2011 #1
    1. The problem statement, all variables and given/known data
    1. How much distance does a car cover during this period: from 60 Km/Hr, a car comes to a dead stop in 10 seconds, from the time the brake is applied (answer in m/s)
    2. What will be the braking distance if the car travels at 90Km/hour (answer in m/s)

    2. Relevant equations
    s = ut + 1/2at^2
    v^2 = u^2 + 2as

    3. The attempt at a solution
    NOTE: What equation is best used for the first question, is it distance=v*t OR one of the complicated distance equations since acceleration needs to be found. I am unsure whether you must retain 60 km/hr as initial velocity to 90 km/hr as final velocity in the 2nd question.
     
  2. jcsd
  3. Jul 20, 2011 #2
    One can't simply use d=vt in this case since the velocity is changing during the braking process.
    It will be necessary to use a "complicated distance equation", however, not one of the two you listed since you don't know the acceleration yet. Most likely you have a list of four basic kinematic equations for motion with constant acceleration. Assuming you do, is there one that does not have a(for acceleration) in it?
    As for the second part, it resets the initial conditions of the problem. The new initial velocity is 90 km/hr and that must be used.
     
  4. Jul 20, 2011 #3
    Thanks for settling the second question but I am still unsure which equation would be best since there is no final velocity and no acceleration.
     
  5. Jul 20, 2011 #4
    Take a look at this post: https://www.physicsforums.com/showthread.php?t=95426
    Click on the link(it's titled "motion_1dim.pdf").
    Scroll down to page 11, you will see four equations of motion. There is one that does not contain acceleration. See if that helps you.

    "...there is no final velocity...". Reread the question and think of what "...dead stop..." means and how you would assign a value to it.

    Hope this helps.
     
  6. Jul 20, 2011 #5
    I think I have an idea, thanks for the reply
     
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