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Homework Help: Calculate the velocity of the boat

  1. Dec 28, 2007 #1
    1. A child in a boat throws a 6.40-kg package out horizontally with a speed of 10.0 m/s. Calculate the velocity of the boat immediately after, assuming it was initially at rest. The mass of the child is 26.0 kg, and that of the boat is 45.0 kg. Ignore water resistance.
    Should I use conservation of momentum to solve it? But here the two objects are package and boat? how about the child?
    2. If the speed of a car is increased by 50%, by what factor will its minimum braking distance be increased, assuming all else is the same? Ignore the driver's reaction time.
    I have no any clue for this one.
    Hope you can give me some hint, thanks.
  2. jcsd
  3. Dec 29, 2007 #2


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    Should I use conservation of momentum to solve it?
    But here the two objects are package and boat? how about the child?
    Here if the child remains at rest while throughing the packet, then you have to consider the combined mass of child and boat.
    In problem 2, calculate the increase in the KE when the speed increases by 50% and see how the KE and braking distance are related
  4. Dec 31, 2007 #3
    I have not considered #2 but for #1, is not time information needed (acceleration of the package / time taken for it to reach 10 m/s) in order to answer the question?
  5. Dec 31, 2007 #4
    Please excuse my cerebral lapse. Of course, velocity can be calculated by dividing momentum over mass. So the momentum of the stone, 6.4 kg ms, over the combined mass of the child and boat would give the boat a velocity of 6.4 / 71 = ~0.09 m/s, ignoring resistance.
  6. Dec 31, 2007 #5
    I think you have a little math error, you've stated the momemtum is 6.4 kg m/s, but you perhaps forgot to multiply the 6.4 kg by 10 m/s?

    Or is it me, I'm a little rusty.
  7. Dec 31, 2007 #6
    Yet another failure on my part. I have lost so many marks through silly errors such as this. The answer, unless I am again mistaken is:

    V = p/m = (6.4 * 10) / 71 = 64 / 71 = ~0.9 m/s
    Last edited: Dec 31, 2007
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