65 Human Stability

Lawrence Davis

When asking what makes a structure more or less stable, we find that a high center of gravity or a small support base makes a structure less stable. In these cases a small displacement is need in order to move the center of gravity outside the area of support. Structures with a low center of gravity compared to the size of the support area  are more stable. One way to visualize stability is to imagine displacement of the center of gravity caused by placing the object on a slope. For example, a 10° displacement angle might displace the center of gravity of a toddler beyond the support base formed by its feet, while an adult would still be in equilibrium.

Left: An adult and toddler walk side-by-side. The gravitational force points downward from the center of gravity of the adult, located at their waist. The force of gravity passes through a line connecting the two feet to indicate the support base width. The gravitational force on the toddler points downward from the center of gravity, located between the shoulder blades. The force of gravity passes through a line connecting the two feet to indicate the toddler support base width. Right: The same diagram is not tilted by an angle of 10 degrees. The force of gravity in each case still points straight down and for the adult still passes through a line connecting their feet to indicate the support base width. The gravitational force on the toddler no longer passes through a line connecting their feet to indicate the support base width.
Compared to an adult, a smaller displacement will move a toddlers center of gravity outside the base of support. Image adapted from A man and a toddler take a leisurely walk on a boardwalk by Steve Hillibrand via Wikimedia Commons.

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The center of gravity of a person’s body is above the pivots in the hips, which is relatively high compared to the size of the support base formed by the feet, so displacements must be quickly controlled. This control is a nervous system function that is developed when we learn to hold our bodies erect as infants.  For increased stability while standing, the feet should be spread apart, giving a larger base of support. Stability is also increased by bending the knees, which lowers the center of gravity toward the base of support. A cane, a crutch, or a walker increases the stability of the user by widening the base of support. Due to their disproportionately large heads, young children have their center of gravity between the shoulders, rather than down near the hips, which decreases their stability and increases the likelihood of reaching a tipping point.[2]

Warning label on a bucket indicating the danger of children falling into a bucket and drowning. This danger is caused by the inherent instability of the toddler body. Image Credit: GodsMoon via Wikimedia Commons.

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Reinforcement Exercises

Davis, Lawrence. Body Physics: Motion to Metabolism. Open Oregon Educational Resources. https://openoregon.pressbooks.pub/bodyphysics

  1. " A man and toddler take a leisurely walk on a boardwalk" by  Steve Hillebrand, U.S. Fish and Wildlife Service,  Wikimedia Commons, is in the Public Domain ↵
  2. OpenStax, College Physics. OpenStax CNX. Aug 3, 2018 http://cnx.org/contents/031da8d3-b525-429c-80cf-6c8ed997733a@12.1. ↵
  3. "Drowning Child Warning" by GodsMoon, Wikimedia Commons is licensed under CC BY-SA 2.0 ↵
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