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Homework Help: PVg- volume weight

  1. Nov 9, 2005 #1
    I have a problem that I can't solve,

    A bathball with the volume 3,0dm^3 is floating on water and has 6,0% of it's volume down in the water.

    a) Count the bathball's mass , Easy : 0,06*3,0= 1,8hg = 0,18 kg

    Now comes the real problem

    b) how much power do you need to use to force the whole bathball down under water?

    Well for this I though i'd use p*v*g /p (where p is water density)
    Water density 1,0 * 10^3 = 1000*0,3m^3*9,82 = 2946 / 1000 = 2,946 N

    Which totaly wrong it should be 28 N how do I solve this?
     
  2. jcsd
  3. Nov 9, 2005 #2

    Päällikkö

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    Draw a diagram. You should have three forces.

    "p*v*g /p"
    How did you get this?
     
  4. Nov 9, 2005 #3
    Eh????

    Don't know what you're suggesting please understand I'm no science freak :cry: :cry: :bugeye: :surprised :rofl: :zzz:

    Yeah I have 3 forces gravity is one my power pressing the bathball down is one and the will to go up is the third. But I do not not the force pressing the item down!?
    And how do I get to know that ?
    Give me a formula or something?
     
  5. Nov 9, 2005 #4

    Päällikkö

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    There isn't much help I can give without solving the problem. Here goes, though:

    Well, the forces should equal zero (for the minimal force).
    So:
    [tex]mg + F = \rho _{water} Vg[/tex]
     
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