# Dimensional analysis and frustum of a cone

• selig

## Homework Statement

Hi
Im having some difficulty with the following question:
Figure P1.14 shows a frustrum of a cone. Of the following mensuration (geometrical) expressions, which describes (a) the total circumference of the flat circular faces, (b) the volume, and (c) the area of the curved surface?
(i) π(r1 + r2)[h2 + (r1 – r2)2]1/2 (ii) 2π(r1 + r2) (iii) πh(r1^2 + r1r2 + r2^2)

and I am at part b.
Since I already know that the volume of a frustum of a cone is number (iii) I have to now prove it.
The problem is that I am having some difficulty showing it with the use of dimensional analysis.

Since V=[L^3]
how is possible that πh(r^12 + r1r2 + r2^2) is equal to it?
I know that r=L and h=L but completely confused on how to set it up
can someone point me in the right direction? I don't

thank you

First start off with a cone, say cone A. If you slice off a smaller cone, say cone B, from the "top" of cone A, the solid that remains is a frustrum.

For this question, I believe $$r_1$$ is the radius of the circular base of cone A (equivalently, the larger flat circular face of the frustrum) and $$r_2$$ is the radius of the circular base of cone B (equivalently, the smaller flat circular face of the frustrum). I think the quantity h refers to the height of the frustrum.

Here are some useful hints for solving part (b) of the question:
1) How do you find the volume of a cone?
2) Cones A and B are similar; use this to express the heights of cones A and B in terms of $$r_1$$, $$r_2$$ and h.
3) Note that $$x^3 - y^3 \ = \ (x-y)(x^2+xy+y^2)$$.

To clarify some of the expressions in your original post,
Option (i) $$\pi(r_1+r_2)\sqrt{h^2+(r_1-r_2)^2}$$
Option (iii) $$\frac{1}{3}\pi h (r_1^2+r_{1}r_{2}+r_2^2)$$

Last edited:
thank you, I think I got it

I'm struggling on practically this exact problem right now in my textbook.

The way I see it, I get this:

L (L2 + L2 + L2) = L3 + L3 + L3 = 3L3

But I don't see how I can arrive at L3 from my end answer...

I'm struggling on practically this exact problem right now in my textbook.

The way I see it, I get this:

L (L2 + L2 + L2) = L3 + L3 + L3 = 3L3

But I don't see how I can arrive at L3 from my end answer...

I just had to answer my own question here. The 3 disappears because it's dimensionless!