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Homework Help: Finding Drag w/ acceleration & Boyant force

  1. Nov 3, 2008 #1
    1. The problem statement, all variables and given/known data
    A speed boat is accelerating at 5 m/s/s. If the Boyant force (the force the prevents the boat from sinking) is 3140 N and the thrust that the motor exerts on the boat is 1876 N, what is the drag on the boat (drag in a fluid is a like friction on land).


    2. Relevant equations
    Fnet = ma


    3. The attempt at a solution
    We just started getting 2 part equations and I'm a little confused on what I have to solve for first. Would someone help me figure out what to solve for first?
    I was thinking that since the boyant force is 3140N going up then gravity must be pulling 3,140N down right? So then the acceleration downwards is 9.81 m/s/s, so 3,140N = X * 9.81 then divide 3,140 by 9.81 and you get 320.081549kg. Does this help at all?
    Fnet = ma
    Fnet = (320kg * 5m/s/s)
    If that is correct, what do I use for Fnet
     
  2. jcsd
  3. Nov 3, 2008 #2

    PhanthomJay

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    Yes looks good so far. The boat is accelerating in the x direction. The net force is the algebraic sum of the forces acting on the boat in the x direction. What are those forces? (one is given, the other is unknown; you need to solve for it).
     
  4. Nov 3, 2008 #3
    1876 N - Ff = (320kg * 5m/s/s)

    1876N/(320kg * 5m/s/s) = Ff

    1.1725 is the force of friction? I'm pretty sure thats not right, what am I missing?
     
  5. Nov 3, 2008 #4

    PhanthomJay

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    Your algebra!
    1876-Ff = 320(5)
    Ff = 1876 - (320)5
     
  6. Nov 3, 2008 #5
    Haha thanks a lot, that was a dumb mistake.
    I appreciate the help!
     
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