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Homework Help: Deceleration without knowing the time

  1. Oct 14, 2014 #1
    1. The problem statement, all variables and given/known data
    What net force would be necessary to stop an automobile of mass 1000kg traveling initially at a speed of 15m/s in a distance equal to the diameter of a dime, which is 1.8cm ?

    2. Relevant equations

    3. The attempt at a solution

    The force required to stop the car is given by F = dp/dt.

    Change in momentum = mv - mv0

    As far as I can see, it's not possible for me to determine a time in which this change in momentum takes place. I have been staring at it for ages and not making any head way.

    Thanks for any help you can give.
  2. jcsd
  3. Oct 14, 2014 #2


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    Staff: Mentor

    You have to introduce distance. What is the relationship between acceleration and distance travelled?
  4. Oct 14, 2014 #3
    Acceleration is the second derivative of displacement with respect to time.
  5. Oct 14, 2014 #4


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    2017 Award

    Staff: Mentor

    That is correct, but not the important point.

    There are two approaches:
    (a) assuming constant deceleration, what is the relationship between initial velocity, acceleration and distance travelled in the stopping process?
    (b) Work with the energy of the car.
  6. Oct 14, 2014 #5
    I was overcomplicating this... a = (v^2 - u^2)/2s

    I have solved the problem.
  7. Oct 14, 2014 #6
    I just realised my mistake before reading your post. I moved on to the next problem and the answer to this one was staring me in the face.
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