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Homework Help: Frictional forces acting on log Application of Newtons Laws

  1. Feb 19, 2008 #1
    A rope is tied to a 97.8 kg log. When you pull on the rope with a horizontal force of magnitude 706 N, the log moves at constant speed.

    a) Find the magnitude of the resistive (frictional) force acting on the log. You may assume the frictional forces acting on the log is constant.

    b) To pull the log with an acceleration of magnitude 1.46 m/s2, what is the magnitude of the horizontal force must you now exert on the rope? Assume the resistive force has not changed.

    This one has really stumped me, I don't know where to begin as my physics class has not even covered friction yet. My professor assigned this as part of an online homework assignment for a lecture he did not even give (due to the recent holidays). He doesn't want to lose time cuz we have a lot of material to cover this semester. This is due tomorrow and it has stumped me all week, someone please HELP!
  2. jcsd
  3. Feb 19, 2008 #2
    For a,

    The sum of the forces in the x direction = ma

    Pulling force + Frictional force = m(0) (remember + or - signs depending on direction, friction acts against movement)

    I think that should help.

    For b, use the same value of the frictional force from part a, then

    Pulling force + Frictional force = ma

    solve for the pulling force...
  4. Feb 19, 2008 #3
    The biggest problem that I had with this problem is that the log moves at constant v, so a=0. How can a force move on object with 0 acceleration? Is this log considered to be in equillibrium even if its moving?
  5. Feb 19, 2008 #4
    so basically the answer is -706 N if i understand you correctly:

    Pulling Force + Friction Force = m (0)

    706 N + f = m (0)

    f = 0 -706 N
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