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Homework Help: Is there anything missing in this motion equation?

  1. Apr 20, 2013 #1
    have found this interesting question (interesting to me).... along with an equation that is supposed to allow the answer..... but I plug in the data and never get the same result as is correct.

    could someone set it out with the data so it flows to give -690N (if correct).... thanks
    (not an exam question or anything.... I found it and would like it as a model to find average brake force.... cheers)

    A 1100-kg car traveling at 27 m/s starts to decelerate and comes to a complete stop in 578.0 m. What is the average braking force acting on the car?
    answer: (a) -690 N (b) -340 N (c) -410 N (d) -550 N

    the answer is said to be (a) -690N
    and the suggested equations are these:

    Use Newton's second law in the form:
    F = ma

    And this equation of motion:
    v2 = u2 + 2as

    m = mass
    F = braking force
    v = final velocity
    u = initial velocity
    s = displacement

    You end up with an equation like this:
    F = (mv2)/2s

    and the answer is -690 N.

    appreciate thoughts, thanks
  2. jcsd
  3. Apr 20, 2013 #2


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    Homework Helper

    F = (mv^2)/2s gives 693 N.

    You could have gotten the same answer by using conservation of energy:

    Loss of KE by car = work done by friction force

  4. Apr 20, 2013 #3
    Thanks.... gotit in end....
    didn't bother with the end equation...... simply used first 2 equations in own right:

    * started with v2=u2+2as
    * then plugged that into F=ma
    * this got me to the answer

    All done
    Thanks for input, much appreciated
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