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Homework Help: Force due to wind and rolling friction on van

  1. Feb 27, 2007 #1
    1. The problem statement, all variables and given/known data
    The graph shows the speed as a function of time for a minivan, coasting on neutral along a straight, level road, loaded with windsurfing equipment and towing a boat. The total mass is 2500 kg, weight= 5512lbs. Find the size of the force due to wind and rolling friction when the van speed is 44.17 mph. (1 mph=0.447m/s)


    2. Relevant equations
    F=ma


    3. The attempt at a solution
    I don't think anyone needs to see the graph to understand my problem. Well I hope.

    I understand that a= delta v/delta t. But whenever I use the equation I get an acceleration that does not work. Then I thought that since the equation wants the force at 44.17 mph, that I need to find the instantaneous acceleration.

    That's my problem: I don't know how to get the instantaneous acceleration. At 44.17 mph the time is at 19 s. Should I create a derivative. If so, how do I go about it?

    The answer for this problem is 764 N

    Please tell me why my approach is incorrect.
     
  2. jcsd
  3. Feb 27, 2007 #2

    That's right, calculate the instantaneous acceleration by finding the derivative of the graph at the given time. You must be able to find a way to do that (draw a tangtent).

    Good luck!
     
  4. Feb 27, 2007 #3
    da_willem, that is my problem. I do not know how to find the derivative of the graph. If I am given a derivative, I can do it; but to look at a graph and create one I just don't know.
     
  5. Mar 2, 2007 #4
    In the graph you draw a line tangential to a point where you want to know the derivative (a 'tangent line'). The derivative in that point is now the 'slope' of this line, i.e. a vertical interval ([itex] \Delta v[/tex]) divided by a corresponding horizontal interval ([itex] \Delta t[/tex]): ([itex] \Delta v /\Delta t[/tex]).
     
  6. Mar 6, 2007 #5
    I got it now. Thank you da_willem
     
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