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Mass of disk given mass per unit

  1. Apr 12, 2013 #1
    1. The problem statement, all variables and given/known data

    I need to find the mass of a disk with a hole, given inner and outer diameters of 0.10m and 0.30m, respectively. The disk has a mass per unit of 20 [itex]\frac{kg}{m^2}[/itex].

    3. The attempt at a solution

    Since I needed mass units, I thought that I could multiply the mass per unit by the area of each disk, the disk as a whole and the missing hole. (The missing will be subtracted from the whole)

    So,

    Whole disk:
    20 [itex]\frac{kg}{m^2}[/itex] × [itex]\pi[/itex] × 0.30 m2 = 5.65 kg

    Missing hole:
    20 [itex]\frac{kg}{m^2}[/itex] × [itex]\pi[/itex] × 0.10 m2 =0.628 kg

    Total mass of disk:
    5.65 kg - 0.628kg = 5.03 kg

    Is this correct?

    I will also have another question that I will need answered related to this but I will try it on my own and provide my calculations.

    Thanks for the help!
     
  2. jcsd
  3. Apr 12, 2013 #2

    SteamKing

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    What is the formula for the area of a circle?
     
  4. Apr 13, 2013 #3
    Area of Circle: [itex]\pi[/itex]r2
     
  5. Apr 13, 2013 #4

    SteamKing

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    And have you used the correct r in your calculations? Hint: Read the OP again very carefully.
     
  6. Apr 13, 2013 #5
    Oh......I used the diameter instead of the radius!!

    20[itex]\frac{kg}{m^2}[/itex] × 0.152 = 1.414 kg

    20[itex]\frac{kg}{m^2}[/itex] × 0.052 = 0.158 kg

    Total mass: 1.414 - 0.158 = 1.256 kg
     
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