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Homework Help: Velocity-Friction proportionality.

  1. Oct 15, 2009 #1
    1. A 1000kg boat is traveling across a lake when its engine is shut off. The magnitude of the frictional force between the water and the boat is proportional to the boat's speed (proportionality constant is 70) . Find the time it takes the boat to slow to one half its speed when the engine was shut off.

    2.Using Newton's second law: F=ma

    3. I'm guessing the acceleration is not constant in this problem. So the constant acceleration equations don't apply. I don't know how to include the time in this problem if i have no equations to solve them in.
    Thanx in advance.
    Last edited: Oct 15, 2009
  2. jcsd
  3. Oct 15, 2009 #2


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    At any instant Fr = 70* v.
    Or retarding force ar = 70/1000*v Or
    -dv/dt = 0.07*v or
    -(dv/v) = 0.07*dt.
    Take the integration between v to v/2 and find t.
  4. Oct 15, 2009 #3
    Thank you so much ! makes perfect sense now !
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