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Upward force rising boat

  1. May 27, 2009 #1
    Can anyone tell me how much upward force a boat (say 24 feet x 8 feet, weighing 4000 lbs) exerts as it rises 1 foot on an incoming tide? My son asked me whether or not somone could hold the boat down as it tried to rise and I thought not but can't quite figure out how to quantify how hard it would be to try. Is it something that would strain a rope if you tried to hold it down (assuming no stretch)? Is "force" even the right term? Should I be asking how much work is done?

  2. jcsd
  3. May 27, 2009 #2


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    Staff: Mentor

    Welcome to the PF. You can read more about bouyancy under the topic of Archimedes Principle:


    The force exerted on a floating object is equal to the weight of the water displaced. So to answer your son's question, you will need to figure out the extra volume of water that gets displaced as you hold the boat in place and the water rises, and then figure out how much that extra volume of displaced water weighs.
  4. May 27, 2009 #3
    If the water were up to the gunnels, a little downward force would sink it.
  5. May 27, 2009 #4


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    Science Advisor

    Similarily you could ask him how much force it would take to "pull" a boat down 1 foot.

    hint: Let's say you have a 12'x4' boat. Then you have 48 cubic feet of displaced water. That's just under 3000 lb of displaced water....
  6. May 28, 2009 #5
    Thanks everyone. So for I guess for my 24x8 example (assuming it's squared-off), that would be about 12,000 lbs of force. That's a lot.

    Thanks again.
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