Results 1 to 5 of 5

Math Help - Thickness of Cylindrical Pipe

  1. #1
    Member
    Joined
    Nov 2009
    Posts
    78

    Question Thickness of Cylindrical Pipe

    The volume of a metallic cylindrical pipe is 748 cu.cm. Its length is 14 cm & its external radius is 9 cm. How do I find its thickness?

    This is what I tried:

    pi x r x r x h = 748
    pi x r x r x 14 = 748
    r x r = 17
    r = 4.17 (this is the internal radius)

    Now thickness = external radius - internal radius
    thickness = 9 - 4.17 = 5.83 cm

    But the answer is wrong. Can someone help me out with this?

    Thanks,

    Ron
    Follow Math Help Forum on Facebook and Google+

  2. #2
    Senior Member
    Joined
    Dec 2010
    Posts
    470
    Let r_{out}, r_{in}, th be the outside radius, inside radius, and thickness respectively.

    Thickness is by:
    th = r_{out} - r_{in}

    The volume is given by (volume of outside minus inside):
    \mathit{Vol} = \pi r_{out}^2h - \pi r_{in}^2h = \pi h(r_{out}^2 - r_{in}^2).

    See what you did wrong?
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    Nov 2009
    Posts
    78
    But why

    \mathit{Vol} = \pi r_{out}^2h - \pi r_{in}^2h

    Volume is the capacity inside the cylindrical pipe..........
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Super Member

    Joined
    May 2006
    From
    Lexington, MA (USA)
    Posts
    11,914
    Thanks
    779
    Hello, rn5a!

    snowtea is absolutely correct . . .


    But why?

    \mathit{Vol} = \pi r_{out}^2h - \pi r_{in}^2h

    Volume is the capacity inside the cylindrical pipe. . . . . no

    "Volume" refers to the amount of material used to make the pipe,
    . . not its "empty space".

    Follow Math Help Forum on Facebook and Google+

  5. #5
    MHF Contributor

    Joined
    Apr 2005
    Posts
    16,454
    Thanks
    1868
    Quote Originally Posted by rn5a View Post
    But why

    \mathit{Vol} = \pi r_{out}^2h - \pi r_{in}^2h

    Volume is the capacity inside the cylindrical pipe..........
    Well, this is your problem. Are you told specifically that the "volume" given refers to the inner capacity of the pipe? the interpretation of snowtea and Soroban, that the volume is the volume of material making up the pipe seems more reasonable to me- especially since by your interpretation, the outer radius is irrelevant. If we call the inner radius "r" then the inner volume of the pipe is just \pi r^2h= 14\pi r^2= 748. From that, r^2= \frac{748}{(3.1416)(14)}= 17.0 so that r= \sqrt{17.0)= 4.12 inches.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 2
    Last Post: October 10th 2010, 07:09 AM
  2. Replies: 6
    Last Post: September 20th 2010, 05:52 PM
  3. Replies: 3
    Last Post: February 15th 2010, 12:53 PM
  4. Replies: 4
    Last Post: October 9th 2009, 01:38 PM
  5. Replies: 8
    Last Post: September 13th 2009, 07:47 PM

Search Tags


/mathhelpforum @mathhelpforum