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Homework Help: Force and motion in Newton's laws

  1. Oct 5, 2011 #1
    A 1000kg boat is traveling at 90 km/h when its engine is shut off. The magnitude of the frictional force fk between boat and water is proportional to the speed V of the boat: fk = 70V where V is in meters per second and fk is in newtons. Find the time required for the boat to slow to 45 km/h.

    Attempted to solve:
    Take upwards as positive for y direction, x + from left to right

    From newton's law 2, we have:
    -fk = ma
    a = -fk/m = -70Vi / m

    (Vi = V initial)

    V = Vi + at
    ---------> t = (V - Vi) /a

    t = - m(V - Vi) / 70Vi (h)
    with m = 1000kg, V = 45km/h, Vi = 90 km/h

    However, my answer is not correct from the book. Can someone point out where I did wrong ? Thanks
  2. jcsd
  3. Oct 5, 2011 #2
    If the acceleration is based on the object's speed is it really constant?
  4. Oct 5, 2011 #3
    Oh I got it, since acceleration is not constant, we can't use all the kinematic equations to solve this problem.....But how are we gonna find acceleration if we only have one equation ?
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