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: Cable Taper

  1. Feb 24, 2006 #1
    I'm doing some research on space elevators and have found a site (http://www.zadar.net/space-elevator/#transverse") which gives some insight into the maths behind the elevator cable. If you click on the link and scroll down, up, whichever direction to equation (2) and a picture of a blue trapezium the guy has found the difference in the volume of the tapering cable with what looks like volume of revolution, but I cant seem to figure out what he's done. He says that it's 'easy to show...'. Am I missing something? I could use a little help. thanks.
    Last edited by a moderator: Apr 22, 2017
  2. jcsd
  3. Feb 24, 2006 #2

    Tom Mattson

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Quick way to show this: You are dealing with finding the volume of a frustum of a cone. The volume [itex]V[/itex] of a cone is [itex]V=\frac{1}{3}Ah[/itex], where [itex]A[/itex] is the area of the base and [itex]h[/itex] is the height.

    You can get the volume of the frustum illustrated in the figure by repeatedly applying the above formula and subtracting volumes, as follows:

    [tex]V=V_{frustum+missing top}-V_{missing top}[/tex]

    Long way to show this: You can calculate the volume with a single integral because the frustum is a surface of revolution. Find an equation for the right boundary of the figure (it's a straight line segment, so that's easy) and solve it for x. Then revolve that line segment about the y-axis and write down an expression for the differential volume. Then integrate over y, and you should get the same expression.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook