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Force due to wind and rolling friction

  1. Oct 19, 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 2450kg (weight = 5402lbs). Find the size of the force due to wind and rolling friction when the van speed is 49.27mph (1mph= 0.447m/s).
    [​IMG]


    2. Relevant equations
    delta v/delta t=a
    F=ma


    3. The attempt at a solution
    I converted the mph to m/s, and I drew a line tangent to the point on the graph where the speed is 49.27 mph. I just can't seem to get the numbers right for some reason. Can someone help me out?
     
  2. jcsd
  3. Oct 19, 2007 #2

    PhanthomJay

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    When you drew the tangent, what did you conclude about the acceleration at that point?
     
  4. Oct 19, 2007 #3
    The acceleration is negative, right. So doesn't that make the force negative? The van reaches 49.27 mph at 11 sec. So what other point should I use to find the slope?
     
  5. Oct 19, 2007 #4

    PhanthomJay

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    Draw the tangent as best you can, acros the whole graph, and see what the slope is by choosing any 2 convenient points to determine the approximate acceleration ( change in v/change in t) between those points. Yes, the force will be negative.
     
  6. Oct 19, 2007 #5
    Then once I get the acceleration, I just multiply by 2450 kg?
     
  7. Oct 19, 2007 #6
    alright, I got it, thanks!
     
  8. Oct 19, 2007 #7

    PhanthomJay

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    Yes, that should do it. Your answer will be approximate because you're just scaling from the graph, unless you were given the exact equation for the graph.
     
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