1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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)


    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted