The center of gravity vs the center of buoyancy in martial arts

This post was made obsolete with by this one: https://plus.google.com/105995174001354763323/posts/GB9BstU3vZZ

I'm about to explain the principle of uniform pressure (that post is here: https://plus.google.com/105995174001354763323/posts/4xz9wNiRovJ). But I need to talk about something else first.

The center of buoyancy (CoB) can be thought of as the average physical location of an object assuming it has uniform mass.  Humans are not uniform on the insides.

The center of gravity (CoG) is the average location of a distribution of mass in space.  It's where the weighted relative position of the distributed mass sums to zero.

So if I were to attach a bunch of styrofoam to my chest, my CoG would change very little but my CoB would change considerably more.  The opposite would ring true if I replaced the styrofoam with a lead weight.

Why is this relevant?  Well your CoG is the thing your internal circuitry is trying to keep under control.  Most of the time in martial arts we talk about a persons center and we really mean the CoB.

Why is this relevant? Well your CoG is the thing your internal circuitry is trying to keep under control. Most of the time in martial arts we talk about a persons center and we really mean the CoB.

The CoB is at the centre of mass of the volume of fluid displaced by an object.

The CoG may be located outside the physical body, as is sometimes the case for hollow or open-shaped objects, such as a horseshoe. It changes when a person moves.

--- references ---

[1] Center of buoyancy, wikipedia http://en.wikipedia.org/wiki/Buoyancy
[2] Center of mass, wikipedia http://en.wikipedia.org/wiki/Center_of_mass
[3] Image credit http://vignette1.wikia.nocookie.net/…/Scn0…/revision/latest…

If you wanted to read a bit more about these ideas, you can check out the concept of metacentric height http://en.wikipedia.org/wiki/Metacentric_height#Metacentre

--- continued/further explanation and notes ---

Human bodies don't have uniform structure on the inside. For example, bones won't have the same mass as tissue. All of these parts contribute to the center of gravity.

Bodies are not uniform on the inside. But what if they were? Well if they were uniform, then the center of gravity and the center of buoyancy would be exactly the same. The CoB is really just the center of mass, if an object was uniform on the insides.﻿

People in martial arts and in shooting etc. talk about 'the center' but this can be confusing, since they mean the CoB. But when you throw somehow, it's their CoG that has to move over the base of support. Moving the CoB has the effect of helping move the CoG over the base of support.

Often center of gravity and center of mass are used to mean the same thing. http://www.quora.com/Whats-the-difference-between-the-centre-of-gravity-and-the-centre-of-mass

Estimated center of mass/gravity (blue sphere) of a gymnast at the end of performing a cartwheel. Notice center is outside the body in this position. http://en.wikipedia.org/wiki/File:CofM.jpg

In mathematics and physics, the centroid or geometric center of a two-dimensional region is the arithmetic mean ("average") position of all the points in the shape. http://en.wikipedia.org/wiki/Centroid
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