Solid Chocolate Puzzle

Geometry Level 1

A solid sphere of chocolate has a volume of 288 π 288\pi . The chocolate is then melted down and reshaped into a solid cube. Assume no chocolate was wasted.

Did the sphere have a greater surface area or does the cube?

Useful equations:

  • Volume of a sphere: V = 4 3 π r 3 V=\frac{4}{3}\pi r^3
  • Volume of a cube: V = a 3 V=a^3
  • Surface area of a sphere: A = 4 π r 2 A=4\pi r^2
  • Surface area of a cube: A = 6 a 2 A=6a^2
The Sphere The Cube Their surface areas were equal

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6 solutions

Consider the sphere: Compute r r from V V . We have

288 π = 4 3 π ( r 3 ) 288\pi =\dfrac{4}{3} \pi (r^3) \implies r = 6 r=6

So the surface area of the sphere is S s p h e r e = 4 π r 2 = 4 ( π ) ( 6 2 ) 452.39 S_{sphere}=4 \pi r^2=4(\pi)(6^2)\approx 452.39

Consider the cube: Compute a a from V V , we have

288 π = a 3 288 \pi=a^3 \implies a 9.67 a\approx 9.67

The surface area of the cube S c u b e = 6 ( 9.6 7 2 ) 561.05 S_{cube}=6(9.67^2) \approx 561.05

The cube has a larger surface area. \color{#D61F06}\large{\boxed{\therefore \text{The cube has a larger surface area.}}}

Stewart Feasby
May 1, 2015

Firstly, we are given that the Sphere is melted down and reshaped into a Cube, with no waste. Therefore their volumes must be the same. So we have (using the equations given in the question): 4 3 π r 3 = 288 π a 3 = 288 π \frac { 4 }{ 3 } \pi { r }^{ 3 }=288\pi \\ { a }^{ 3 }=288\pi Therefore we can say that the radius of the sphere and the side-length of the cube is: r = 6 a = 9.67 r=6\\ { a }=9.67 Therefore, using the formulas given, we can calculate the surface areas to be: A s p h e r e = 4 π r 2 = 4 3.14 6 2 = 452 c m 3 A c u b e = 6 a 2 = 6 9.6 7 2 = 561 c m 3 { A }_{ sphere }=4\pi r^{ 2 }=4\cdot 3.14\cdot 6^{ 2 }=452cm^{ 3 }\\{ A }_{ cube }=6a^{ 2 }=6\cdot 9.67^{ 2 }=561cm^{ 3 } Therefore the cube has a greater surface area than the sphere, so the correct answer to the question is: T h e C u b e \boxed {The \ Cube}

Herald Jesalva
Dec 17, 2015

Obviously, the cube has more surface area as spheres have the least amount of surface area to pack a certain volume, as we see in soap bubbles. (We can't have a bubble with a shape other than sphere)

Excellent solution! I love the elegance of it - relating it to packing problems!

Stewart Feasby - 5 years, 5 months ago

volume of sphere = 288π

4/3 * π * r^3 = 288π

r = 6 units

surface area of a sphere = 4 * π * r^2 = 4 * π * 6^2 = 452.39 square units

volume of cube = volume of sphere = 288π

s^3 = 288π

s = 9.67 units

surface area of cube = 6 * s^2 = 6 * 9.67^2 = 561.05 square units

surface area of cube > surface area of sphere

Lu Ca
Jun 20, 2019

It's significant to consider the Area - Volume ratios of the sphere and cube:

A/V ratio of the sphere is:

A s f V s f = 4 π r 2 4 3 π r 3 = 3 r \frac{Asf}{Vsf}=\frac{4\pi r^{^{2}}}{\frac{4}{3}\pi r^{3}} = \frac{3}{r} ;

while

A/V ratio of the cube is:

A c V c = 6 l 2 l 3 = 6 l \frac{Ac}{Vc}=\frac{6l^{2}}{l^{3}}=\frac{6}{l}\ Equation (1).

Volumes are equal so

4 3 π r 3 = l 3 l = r ( 4 3 π ) 1 3 \frac{4}{3}\pi r^{3}=l^{3}\Rightarrow l=r\left (\frac{4}{3}\pi \right )^{\frac{1}{3}} Equation (2) .

So the A/V ratio of the cube EQ (1) can be written as a function of r using Equation 2:

A c V c = 6 ( 4 3 π ) 1 3 r 3.72 r \frac{Ac}{Vc}={\frac{6}{\left (\frac{4}{3}\pi \right )^{\frac{1}{3}}r}}\approx \frac{3.72}{r} .

So if Vc = Vsf then Ac = 1.24 Asf, approximately .

Tina Sobo
Sep 6, 2016

Mathematically, the cube has a higher surface area by the equations in other answers. However, the problem is flawed. There had to have been chocolate that accidentally didn't make it into the cube - it's a law of cooking with chocolate, some ends up in your stomach.

volume of sphere = 288π

4/3 * π * r^3 = 288π

r = 6 units

surface area of a sphere = 4 * π * r^2 = 4 * π * 6^2 = 452.39 square units

volume of cube = volume of sphere = 288π

s^3 = 288π

s = 9.67 units

surface area of cube = 6 * s^2 = 6 * 9.67^2 = 561.05 square units

surface area of cube > surface area of sphere

A Former Brilliant Member - 4 years, 7 months ago

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