Square-cube law

The square-cube law was first mentioned in Two New Sciences (1638).

The square-cube law (or cube-square law) is a principle, drawn from the mathematics of proportion, that is applied in engineering and biomechanics. It was first demonstrated in 1638 in Galileo's Two New Sciences. It states:

When an object undergoes a proportional increase in size, its new volume is proportional to the cube of the multiplier and its new surface area is proportional to the square of the multiplier.

Represented mathematically:

$v_2=v_1\left(\frac{\ell_2}{\ell_1}\right)^3$

where v₁ is the original volume, v₂ is the new volume, $\ell_1$ is the original length and $\ell_2$ is the new length. Which length is used does not matter.

$A_2=A_1\left(\frac{\ell_2}{\ell_1}\right)^2$

where A₁ is the original surface area and A₂ is the new surface area.

For example, a cube with a side length of 1 meter has a surface area of 6 m² and a volume of 1 m³. If the dimensions of the cube were doubled, its surface area would be increased to 24 m² and its volume would be increased to 8 m³. This principle applies to all solids.

Applications

Engineering

When a physical object maintains the same density and is scaled up, its mass is increased by the cube of the multiplier while its surface area only increases by the square of said multiplier. This would mean that when the larger version of the object is accelerated at the same rate as the original, more pressure would be exerted on the surface of the larger object.

Let us consider a simple example of a body of mass, M, having an acceleration, a, and surface area, A, of the surface upon which the accelerating force is acting. The force due to acceleration, F = Ma and the thrust pressure, $T = \frac{F}{A} = M\frac{a}{A}$.

Now, let us consider the object be exaggerated by a multiplier factor = x so that it has a new mass, M' = x³M, and the surface upon which the force is acting has a new surface area, A' = x²A.

The New force due to acceleration F' = x³Ma and the resulting thrust pressure,

$\begin{align} T' &= \frac{F'}{A'}\\ &= \frac{x^3}{x^2} \times M\frac{a}{A}\\ &= x \times M \frac{a}{A}\\ &= x \times T\\ \end{align}$

Thus, just scaling up the size of an object, keeping the same material of construction (density), and same acceleration, would increase the thrust by the same scaling factor. This would indicate that the object would have less ability to resist stress and would be more prone to collapse while accelerating.

This is why large vehicles perform poorly in crash tests and why there are limits to how high buildings can be built. Similarly, the larger an object is, the less other objects would resist its motion, causing its deceleration.

Engineering Examples

• Airbus A380: the lift and control surfaces (wings, rudders and elevators) are relatively big compared to the fuselage of the airplane. In e.g. a Boeing 737 these relationships seem much more 'proportional', but designing an A380 sized aircraft by merely magnifying the design dimensions of a 737, would result in wings that are too small for the aircraft weight, because of the square-cube rule.
• A clipper needs relatively more sail surface than a sloop to reach the same speed, meaning there is a higher sail-surface-to-sail-surface ratio between these crafts then there is a weight-to-weight ratio.

Biomechanics

If an animal were scaled up by a considerable amount, its relative muscular strength would be severely reduced, since the cross section of its muscles would increase by the square of the scaling factor while its mass would increase by the cube of the scaling factor. As a result of this, cardiovascular and respiratory functions would be severely burdened.

In the case of flying animals, the wing loading would be increased if they were scaled up, and they would therefore have to fly faster to gain the same amount of lift. Air resistance per unit mass is also higher for smaller animals, which is why a small animal like an ant cannot be crushed by falling from any height.

As was elucidated by J. B. S. Haldane, large animals do not look like small animals: an elephant cannot be mistaken for a mouse scaled up in size. The bones of an elephant are necessarily proportionately much larger than the bones of a mouse, because they must carry proportionately higher weight. To quote from Haldane's seminal essay On Being the Right Size, "...consider a man 60 feet high...Giant Pope and Giant Pagan in the illustrated Pilgrim's Progress.... These monsters...weighed 1000 times as much as Christian. Every square inch of a giant bone had to support 10 times the weight borne by a square inch of human bone. As the human thigh-bone breaks under about 10 times the human weight, Pope and Pagan would have broken their thighs every time they took a step."

The giant animals seen in horror movies (e.g., Godzilla or King Kong) are also unrealistic, as their sheer size would force them to collapse. However, it's no coincidence that the largest animals in existence today are giant aquatic animals, because the buoyancy of water negates to some extent the effects of gravity. Therefore, sea creatures can grow to very large sizes without the same musculoskeletal structures that would be required of similarly sized land creatures.

See also

• Biomechanics
• Allometric law
• On Being the Right Size, an essay by J. B. S. Haldane that considers the changes in shape of animals that would be required by a large change in size

References

• Wayne Throop. "Sauropods, Elephants, Weightlifters: Miscellaneous Issues". http://www.talkorigins.org/faqs/sauropods/sauropods-misc.html. 
• "World Builders: The Limits to Animal Size - Size to Volume Ratio". http://curriculum.calstatela.edu/courses/builders/lessons/less/les9/area.html. 
• Michael C. LaBarbera. "The Biology of B-Movie Monsters". http://fathom.lib.uchicago.edu/2/21701757/.