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Homework Help: Buoyant force/density problem

  1. Mar 3, 2009 #1
    1.A beach ball has a volume ofHow much force would you have to exert to hold this beach completely under water?

    I learned that for an object that is completely submerged in a liquid, the density of the object is equal to the density of the liquid.

    I used the formula p=m/v to get the mass of the ball. I set p as the density of water which is 1000 kg/m^3. The answer I got was 50 kg. Then I multiply it to 9.8 m/s^2 to get the buoyant force.

    My answer was 490 N...

    Am I correct???
  2. jcsd
  3. Mar 3, 2009 #2


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    No. The buoyant force on the object is equal to the weight of the liquid displaced. Holding the beach ball under water replaces the volume of water with the volume of air. The difference in the weight of the air and the water displaced is what gives the ball lift.

    If this is what you are doing with your calculation, (I can't see your volume), then you have the right method, if only the wrong expression of what you are doing.
  4. Mar 3, 2009 #3
    ooops.. sorry I forgot.

    The volume of the ball is 0.050 m^3..

    Is my answer correct???
  5. Mar 3, 2009 #4


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    .05 m³ is 50 kg is 490 N Looks ok then.
  6. Mar 3, 2009 #5
    Thanks for the help sir...!
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