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: Sphere inside a cone question

  1. Sep 18, 2011 #1
    1. The problem statement, all variables and given/known data

    A cone is circumscribed around a sphere. The radius of the sphere is 5 units.
    Write the volume of the cone in terms of x.
    There is a diagram.. I will try to describe it:
    It is a cross section of the object (sphere in cone). From the center of the circle to the bottom left vertice of the triangle is length 5 + x. (5 is the radius, x is the rest of the line).

    2. Relevant equations

    3. The attempt at a solution

    So I can get r of the cone with Pythagoras = sqrt((x + 5)^2 - 25). The height would be 10 + something. I'm not sure how the missing part of the height relates to x.
  2. jcsd
  3. Sep 18, 2011 #2


    User Avatar
    Science Advisor

    The problem is that there are an infinite number of such cones. First draw a triangle circumscribing a circle (the sphere inside the cone seen from the side). Choose and angle for the vertex at the top of the sphere. That angle cannot be 0 or 180 degrees but it can be any other between. And then there exist a cone, having that angle at the vertex, circumscribing the sphere.
  4. Sep 18, 2011 #3
    So how can I write the volume in terms of x ?
  5. Sep 19, 2011 #4


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member


    I certainly wouldn't have chosen x as the main variable in this problem, but never mind that. Look at the figure. All you need to find the volume of the cone is its radius and height. So in the figure you need to get r and y in terms of x. r is easy from the right triangle AOB. Then you can get w + v in terms of y and x from triangle ABD. Then use the similarity of triangles ABD and DCO to get y in terms of x.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook