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Homework Help: Weight of sphere underwater?

  1. Jul 13, 2009 #1
    1. The problem statement, all variables and given/known data

    A hollow sphere has average density of 3 g/cm^3 and a mass of 120g. What will the sphere weight under water?

    correct answer: 0.8

    2. Relevant equations



    3. The attempt at a solution

    I converted 120g into kg and then used w=mg to find weight on land. however, this does get 0.8N underwater. Please help thanks!
  2. jcsd
  3. Jul 13, 2009 #2
    first, change all the units to SI units, giving you:
    3000 kg/m3 for the sphere's density
    0.12 kg for its mass

    to possibly calculate the sphere's weight underwater, you must calculate the net force of the sphere's weight downwards and buoyancy upwards

    buoyant force is equal to the weight of the water displaced by the ball, thus you calculate the weight by using the mass of the water displaced, found by:
    1. density of water
    2. the volume of water displaced, which is equal to the volume of the sphere since it is fully submerged
  4. Jul 13, 2009 #3
    got it, thanks!!
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