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Homework Help: Theorems of Pappus (Finding mass)

  1. Mar 20, 2015 #1
    1. The problem statement, all variables and given/known data
    A steel die, shown in section, has the form of a solid
    generated by revolving the shaded area around the
    z-axis. Calculate the mass m of the die.


    2. Relevant equations

    V =θ(rA) where r is the distance from the origin to the centroid of each shape

    m = ρV where ρ = 7830 kg/m3

    3. The attempt at a solution

    Obviously θ = 2π.

    If I make rA = ( r1A1 - r2A2 ), then A1 = area of right most rectangle = (100 mm x 200 mm) and r1 = distance from origin to that rectangle's centroid = 60 mm + ( 100 mm / 2 ) = 110 mm.

    Also, A2 = area of half-circle = π(R2)/2 = π(602)/2 ≅ 5654 mm and r1 = distance from origin to that half circle = 60mm + 4R/(3π) ≅ 85.46 mm.

    This method gives me correct volume (according to answer given) which in turn gives me correct mass m ≅ 84.5 kg. However, how come I don't have to take into account the rectangle of area (60mm x 200mm) (which is to the left of the half circle) along with its coordinate from origin to centroid (which would be 30mm)? Wouldn't I have to make that r3A3 and subtract that from ( r1A1 - r2A2 ) ?
  2. jcsd
  3. Mar 20, 2015 #2


    User Avatar

    Staff: Mentor

    Because that area is part of the void, it's empty. The question asks you to calculate that volume shown cross-hatched.
  4. Mar 21, 2015 #3
    By "shaded area", they mean the cross-hatched section right? Wouldn't that make the half circle area void as well?

    edit: never mind I get what you're saying. Thanks lol. The volume of only the cross hatched section means obtaining that area and coordinate only. Which means I have to subtract the half circle area and its coordinate. OK, I guess I thought the problem wanted the whole figure not just the cross hatched section. Thanks again
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