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Homework Help: Help? I don't get this at all.

  1. Feb 7, 2007 #1
    1. The problem statement, all variables and given/known data

    A perfectly spherical piece of metal is found at the bottom of a wishing well. The mass of the object is 0.65 kg and the radius is .25 m. What would be it's weight if it had the same volume and were made of pure gold? The density of pure gold is 19,3000 kg/m^3. Answer in units of N.

    2. Relevant equations

    volume = (4 Pi radius^3) / 3

    3. The attempt at a solution

    volume = (4 pi .25^3)/3
    volume = 0.06544984694978735
    volume * 19,300 = 12631.82046

    My main issue is the units of N. Could someone help me with what exctly this is and how to convert to it?

    I'm just looking for tips, and then I can attempt the problem and post again if I have more issues!
    Thank you!

    Sorry. I'm new to physics and this just confuses me.
    Help would be great, thanks!
    Last edited: Feb 7, 2007
  2. jcsd
  3. Feb 7, 2007 #2


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    Homework Helper

    note that you have been given radius of the sphere.. hence you know the volume and you know density of gold (given). can then find mass and if you know the acceleration due to gravity... then you can work everything out...
  4. Feb 7, 2007 #3
    almost there. density is how much an arbitrary measure weighs, it could be 20 # per teaspoon, You have 1/2 teaspoon it weighs 10 #. Here you have about .06 meter^3. Youre given what it "weighs" per cubic meter. Then since this is mass not weight, multiply by 9.8 as suggested above to convert "mass" to "weight" as masse is an inherent property, while weight is particular to situations when doing things with it up or down against gravity.
    Last edited: Feb 7, 2007
  5. Feb 7, 2007 #4
    Yeah but what is the equation for newtons? I need my ending unit to be Newtons.
    I can work it out it's just I need the equation to finish it.
    Last edited: Feb 7, 2007
  6. Feb 7, 2007 #5
    I tried to submit the answer
    12631.82046 * 9.8 which equals = 123791.8405

    Isn't that in units of N?

    F = mass * acceleration is what I used.

    But it rejected the answer. (We use a HW website)
    So where did I go wrong?
  7. Feb 7, 2007 #6
    Right so compute the mass by multiplying volume by density. Then multiply by 9.8 to convert mass to Newtons.
  8. Feb 7, 2007 #7
    Argh! Nevermind.
    I had acidentally slipped the decimal place into the wrong spot.
    Thanks everyone for you help! Finally got it right.
  9. Feb 7, 2007 #8
    Missed adecimal point I think, try again.
  10. Feb 8, 2007 #9
    Mass= Volume*Density
    (We use the kg system here .. cant say about lbs!)

    but I wonder how is mass given in Kg?? mass must be in Newtons.. aint'it? or am drunk at the moment?
    Last edited: Feb 8, 2007
  11. Feb 8, 2007 #10
    Could be but if you're lurching in any direction, its in newtons.
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