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The “G-force” as it is called,  determines the force imparted to objects, depending on the available gravity. You may have heard that astronauts and fighter pilots go through rigorous and vomit-inducing training to combat this, but how has nature dealt with this problem? If the fighter pilot can just barely handle 10 G’s, how could a woodpecker withstand 1200?!

What we will explore below is some facts and STATA (stats and data) on G-forces humans have subjected ourselves to, the basic reasoning and math behind the “G-force”, and marvel at how natural selection has produced, in effect, the world’s best shock absorber.

### Humans and Gravity

So what does a “G” feel like? Well actually you are feeling it right now. When you are there, sitting and looking at the screen, you are experiencing 1 G. You don’t notice it because that is the weight-force that we have evolved to endure. I use the term “weight-force” because, as will be explained below in the “Physics” section, our mass remains constant and the gravity changes.

Every bit of our muscular-skeletal system is naturally crafted to deal with Earth’s gravity. Were you to travel to a planet that had a more significant gravity, a more massive planet with 5 G’s for example, you would either be unable to lift yourself off the ground, our the new weight-force would be so much that the air would be forced out of your lungs and your eyes would explode (something gruesome for sure).

My point is that we are very well-equipped to deal with our own planet’s gravity, or 1 G, and the only time we notice this force is when it becomes higher or lower than 1 G. Examples are easy to find. When you are traveling by plane and you take off, the plane accelerates to very high speeds in order to do so. This rapid increase in velocity effectively changes the G-force on your body. Also, if you have ever been on a roller-coaster, and you travel into one of the sharp valleys on the coaster, you feel like something is pushing your head down, or pressing into your chest right? That’s the G-force making you feel heavier (you of course don’t actually get heavier).

Of course, the opposite is also noticeable. Right before the coaster causes gravity to push down more on your head or chest (traveling upwards), you feel that stomach-in-your-throat feeling (traveling downwards). Because you are effectively in a controlled free-fall, there is a reduction in the G-force on your body, and you feel lighter.

### The Data:

• Astronauts aboard a Space Shuttle reach about 3.5 G’s.
• Amusement park riders on roller coasters pull various levels of G’s, but they rarely get much beyond 4 G’s.
• Airline passengers aboard a commercial airline reach about 1.5 G’s.
• Professional pilots like the Blue Angels can top 6 G’s. Under special circumstances (and in training) they can approach up to 10 G’s.

### Medical Effects of the G-force:

G-force acclimatization testing with a rocket sled

As you pull more Gs, your weight-force increases correspondingly. At 9 G’s, your 10-pound head will weigh 90 pounds. If you continue to pull high Gs, the G force will push the blood in your body towards your feet and resist your heart’s attempts to pump it back up to your brain. You will begin to get tunnel vision, then things will lose color and turn white, and finally everything will go black. You’ve just experienced the onset of Gravity Induced Loss of Consciousness (GLOC).

From a medical standpoint, at 4G’s, you will start to lose color vision, which is why it is called “graying out”; 4.5 G’s and you may lose vision all together. Higher G’s and your lungs start to collapse, your esophagus stretches, your stomach drops and blood pools significantly in your legs.

### The Physics

Why then, do things like roller coasters and fighter jets cause such a phenomena? It’s simple physics…

Isaac Newton famously developed probably the most important equation in all of physics:

F=m*a

• F=force
• m=mass
• a= acceleration

But because we are taking about the effects of a weight-force, we must also consider the equation for weight:

W=m*g

• W= weight
• m=mass
• g=acceleration due to gravity or G’s

The increased outward acceleration (or G's) of this carnival ride creates a larger centripetal "force" on your body. Because there is a seat behind you to counteract this inertial tendency to move outward, you feel pressed up against it.

We can see then that the weight force is simply an application of the first equation, with the force defined as weight because of the acceleration that we all face every second of every day due to gravity is constant, so it is a definition of convenience. Because our mass doesn’t change (you can get fatter or lighter but for this experiment we will consider it constant when traveling on a jet or coaster), and the acceleration due to gravity is relatively constant, we call this constant force applied to us our weight.

So then let’s take an example. When a fighter pilot accelerates at high speeds upward, why does he feel heavier? Looking at our first equation we can see that the force on our pilot is equal to the mass of that pilot times his/her acceleration. Because weight is normally defined with the acceleration due to gravity (9.81 m/s/s or 32.2 ft/s/s) and a constant mass, when the pilot accelerates faster than the value of gravitational acceleration, the weight-force must change (because the mass does not).

Let’s put some numbers to this:

The pilot accelerates at a rate of 161 ft/s/s or 5 G’s. If that pilot weighed 100 pounds before acceleration, during the acceleration the pilot would weigh effectively (on a scale) 500 pounds! And this is not just represented in the math, the pilot would experience a force on his/her body of 500 pounds, and this is why it is so dangerous for your delicate biological processes.

All that being said, woodpeckers are still the kings of the G-force…

### Woodpeckers Don’t Get Concussions

[Well, unless you were trying to really kill the bird of course, but in the course of their head-banging lives, they do not]

A woodpecker’s head experiences decelerations of 1200g as it drums on a tree at up to 22 times per second. Humans are often left concussed if they experience 80 to 100g, so how the woodpecker avoids brain damage is amazing, but perfectly accounted for by evolution and natural selection.

So Sang-Hee Yoon and Sungmin Park of the University of California, Berkeley, studied video and CT scans of the bird’s head and neck and found that it has four structures that absorb mechanical shock. These are its hard-but-elastic beak; a sinewy, springy tongue-supporting structure that extends behind the skull called the hyoid; an area of spongy bone in its skull; and the way the skull and cerebrospinal fluid interact to suppress vibration.

In reality, nature has created the perfect shock absorber, many times more efficient than our best attempts to create one. It may seem amazing how a small bird can withstand such gargantuan cranial trauma, but this is expected when viewed in the lens of evolution. Would you expect their adaptations to be any less honed if they were cultivated, through successive generations of beneficial mutations, by tens of millions of years worth of evolutionary feedback?

What’s more, a deceleration of 1200 G is equivalent to, over 1 second, coming to a complete stop from 26,000 miles per hour! Can you imagine slamming into a brick wall at that speed? You could escape the Earth’s orbit going that fast, and you would quite literally vaporize to a pink mist on impact. But nature does it, albeit on a much smaller scale; evolution can conquer such heights.

### Adaptations [via The Naked Scientist]

Yeah, they can burrow into concrete

There are a number of woodpecker-specific adaptations which make the practice of repeatedly slamming your head against a hard surface slightly more tolerable.

Firstly, woodpeckers have relatively small brains which, in contrast to a human, are packed fairly tightly inside their skull cavity. This prevents the excessive movement of the brain inside the skull which causes so-called ‘contre-coup’ injuries (literally brain bruising) in humans. These occur when the brain bashes into the skull following a knock on the head. In other words the head stops, but the brain keeps on moving.

Also, because the brain is small it has a high surface area to weight ratio, meaning that the impact force is spread over a much larger area, relatively speaking, compared with a human. Again, this minimizes the applied trauma.

Finally, the woodpecker always ensures that he strikes his target in a dead straight line. This approach avoids placing rotational or sheering stresses on the nerve fibers in the brain. Humans involved in car accidents frequently develop the symptoms of ‘diffuse axonal injury’ or DAI, where sudden deceleration coupled with rotation literally twists the different parts of the brain off each other like a lid coming off a jar. By hammering in a dead straight line woody woodpecker avoids giving himself DAI, further minimizing the risk of brain damage. Such an approach may have implications for the design of protective head gear – such as crash helmets – which could be modified to prevent rotational injuries.

Unfortunately, we’re just not adapted to beat our heads against walls, trees, or even paving slabs, with half the impunity of a woodpecker. Definitely a case of “don’t try this at home”.

Another instance of how amazing nature is, and how rewarding the exploration of physics/physiology/biology/science can be!