MHB Max Volume of Cylinder Inscribed in a Cone: Dimensions?

MarkFL
Gold Member
MHB
Messages
13,284
Reaction score
12
Here is the question:

A cylinder is inscribed in a right circular cone of height 6 and base radius equal to 4.5. What are the dimens?


A cylinder is inscribed in a right circular cone of height 6 and base radius equal to 4.5. What are the dimensions of such a cylinder which has maximum volume?

I have posted a link there to this topic so the OP can see my work.
 
Mathematics news on Phys.org
Hello halle,

Let's work this problem in general terms so that we then derive a formula into which we can plug the given data.

First, let's draw a diagram of a cross-section through the axes of symmetry for the cone and cylinder. We will orient our coordinate axes such that the origin as at the center of the circular base of the cone. The radius and height of the cone are $R$ and $H$ respectively, while the radius and height of the cylinder are $r$ and $h$ respectively. Since these parameters and variables represent linear measures, we assume they are non-negative. Both measures of the cone must be positive.

View attachment 1629

Now, our objective function, that function that we wish to optimize, is the volume $V$ of the cylinder:

$$V(h,r)=\pi hr^2$$

As we can see, the point $(h,r)$ is constrained to lie along the line:

$$y=-\frac{H}{R}x+H$$

and thus, our constraint may be given as:

$$g(h,r)=\frac{H}{R}r-H+h=0$$

Let's first use a single variable approach. We may solve the constraint for $h$:

$$h=H-\frac{H}{R}r=\frac{H(R-r)}{R}$$

Substituting for $h$ into the objective function, we obtain the volume of the cylinder in terms of its radius only (recall the radius and height of the cone are parameters, not variables, and so we treat them as constants):

$$V(r)=\pi\left(\frac{H(R-r)}{R} \right)r^2=\frac{\pi H}{R}\left(Rr^2-r^3 \right)$$

Next, we want to differentiate this with respect to $r$ and equate the result to zero to find the critical value(s).

$$V'(r)=\frac{\pi H}{R}\left(2Rr-3r^2 \right)=\frac{\pi H}{R}r(2R-3r)=0$$

Hence, we find two critical values:

$$r=0,\,\frac{2}{3}R$$

To determine the nature of the extrema associated with these critical values, we may use the second derivative test. Computing the second derivative of the volume function with respect to $r$, we find:

$$V''(r)=\frac{\pi H}{R}\left(2R-6r \right)=\frac{2\pi H}{R}\left(R-3r \right)$$

Thus, we find:

$$V''(0)=2\pi H>0$$

Thus the critical value $r=0$ is at a minimum.

$$V''\left(\frac{2}{3}R \right)=\frac{2\pi H}{R}\left(R-3\left(\frac{2}{3}R \right) \right)=-2\pi H<0$$

Thus the critical value $r=\frac{2}{3}R$ is at a maximum. This is the critical value in which we are interested. Thus the dimensions of the cone which maximize its volume are:

$$r=\frac{2}{3}R$$

$$h=\frac{H\left(R-\frac{2}{3}R \right))}{R}=\frac{1}{3}H$$

We may also use a multi-variable technique: Lagrange multipliers. Using this method, we obtain the system:

[math]2\pi rh=\frac{H}{R}\lambda[/math]

[math]\pi r^2=\lambda[/math]

This implies:

[math]2\pi rh\frac{R}{H}=\pi r^2[/math]

Observing that $r=0$ is the critical point associated with the minimal volume, we may divide through by $\pi r$ to obtain:

[math]2h\frac{R}{H}=r[/math]

Now, substituting for $r$ into the constraint, we find:

[math]h+\frac{H}{R}\cdot2h\frac{R}{H}-H=0[/math]

[math]h=\frac{H}{3}\implies r=\frac{2}{3}R[/math]

Hence, from both methods, we find the maximum volume of the cylinder is:

[math]V_{\max}=V\left(\frac{2}{3}R,\frac{H}{3} \right)=\pi\left(\frac{2}{3}R \right)^2\left(\frac{H}{3} \right)=\frac{4}{27}\pi R^2H[/math]

This is $\dfrac{4}{9}$ the volume of the cone.

Now we may plug in our given data:

$$H=6,\,R=\frac{9}{2}$$

to obtain for this problem:

$$r=\frac{2}{3}\cdot\frac{9}{2}=3$$

$$h=\frac{1}{3}\cdot6=2$$

Thus, we have found that the dimensions of the cylinder inscribed within the given cone having maximum volume are a radius of 3 units and a height of 2 units.
 

Attachments

  • halle.jpg
    halle.jpg
    6.8 KB · Views: 116
Insights auto threads is broken atm, so I'm manually creating these for new Insight articles. In Dirac’s Principles of Quantum Mechanics published in 1930 he introduced a “convenient notation” he referred to as a “delta function” which he treated as a continuum analog to the discrete Kronecker delta. The Kronecker delta is simply the indexed components of the identity operator in matrix algebra Source: https://www.physicsforums.com/insights/what-exactly-is-diracs-delta-function/ by...
Fermat's Last Theorem has long been one of the most famous mathematical problems, and is now one of the most famous theorems. It simply states that the equation $$ a^n+b^n=c^n $$ has no solutions with positive integers if ##n>2.## It was named after Pierre de Fermat (1607-1665). The problem itself stems from the book Arithmetica by Diophantus of Alexandria. It gained popularity because Fermat noted in his copy "Cubum autem in duos cubos, aut quadratoquadratum in duos quadratoquadratos, et...
I'm interested to know whether the equation $$1 = 2 - \frac{1}{2 - \frac{1}{2 - \cdots}}$$ is true or not. It can be shown easily that if the continued fraction converges, it cannot converge to anything else than 1. It seems that if the continued fraction converges, the convergence is very slow. The apparent slowness of the convergence makes it difficult to estimate the presence of true convergence numerically. At the moment I don't know whether this converges or not.

Similar threads

Replies
3
Views
1K
Replies
4
Views
1K
Replies
2
Views
2K
Replies
2
Views
3K
Replies
1
Views
3K
Replies
2
Views
4K
Back
Top