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: Fluids (Density)

  1. Oct 10, 2011 #1
    26 ••• Since 1983, the US Mint has coined pennies that are made out of zinc
    with a copper cladding. The mass of this type of penny is 2.50 g. Model the penny
    as a uniform cylinder of height 1.23 mm and radius 9.50 mm. Assume the copper
    cladding is uniformly thick on all surfaces. If the density of zinc is 7140 kg/m3
    and that of copper is 8930 kg/m3, what is the thickness of the copper cladding?

    The problem is similar to a coin inside a coin. We need to find the outer coin's height- the inner coin's height or thickness of the outer coin.

    The inner coin is made of Zinc.

    m = (ρ Cu)Vcu + (ρ Zn) Vzn, where p is rho of Cu and Zn respectively and Vcu and Vzn are volumes of copper and zinc respectively.

    m = ρ Cu[ 2π(r)square d + 2πr (h − 2d )d ] + ρ Zn π (r − d )(square) (h − 2d ), where d is the thickness sought.

    Could someone help? I don't undestand why the V covering the zinc coin is, what's in bold.
    The volume I get is total volume- volume of zinc coin: πr(square)h-π(r-d)(square)(h-2d)
  2. jcsd
  3. Oct 10, 2011 #2


    User Avatar
    Homework Helper

    one of the Cu π(r)square d is the top disk , the other π(r)square d is the bottom disk.
    2πr (h − 2d) d is the edge band , in between the top and bottom disks "approximately".
    Your way is exact , so it's better ... if you can solve it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook