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Jesse Eaton





Joined: 15 Feb 2008

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PostPosted: Fri 12 Mar, 2010 3:41 pm    Post subject: A Beginner's Glossary of Terms: A point about definition         Reply with quote

I wanted to make a small point about one of the definitions listed in A Beginner's Glossary of Terms. This feature gives the following definition:

Point of Balance (PoB)
The Point of Balance on a sword is simply the point on which the center of gravity is located. In other words, it's the spot along the blade's length that has equal mass on either side of it. The PoB will vary widely between sword types and their intended functions.

The statement "it's the spot along the blade's length that has equal mass on either side of it." and the statement "the point on which the center of gravity is located". are not necessarily compatible. The problem is that a sword may have a center of gravity that is not the same as the spot along the blade that has equal mass on either side of it. The definition listed here only applies to swords that are symmetrical and strait. The center of gravity of a curved blade is rarely on the blade itself. Furthermore, the center of gravity of complex hilted weapons is also often askew from the centerline of the blade so equating the point of balance with the CoG is a bit misleading and missing the point, pun intended Happy .

I think that the PoB is rightly defined as "the spot along the blade's length that has equal mass on either side of it." But center of gravity, or center of mass, needs to be defined separately as the point at which mass is evenly distributed.

Just a small point, but it makes a difference when thinking about and examining a weapon and its use. This distinction might come in even more handy when examining hafted weapons.
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Timo Nieminen




Location: Brisbane, Australia
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PostPosted: Fri 12 Mar, 2010 4:41 pm    Post subject: Re: A Beginner's Glossary of Terms: A point about definition         Reply with quote

Jesse Eaton wrote:

I think that the PoB is rightly defined as "the spot along the blade's length that has equal mass on either side of it." But center of gravity, or center of mass, needs to be defined separately as the point at which mass is evenly distributed.


There is a unique technical definition of centre-of-mass and centre-of-gravity (which are exactly the same thing). And it isn't the point with equal mass on either side. The definition at http://en.wikipedia.org/wiki/Center_of_mass at the time of this post is correct.

Exactly the same definition is also used to mean point-of-balance, but this isn't a unique definition, and this is for the reason you pointed out: the CoG/CoM isn't necessarily on the blade. If we insist that the PoB lie on the weapon itself, then we have the implicit definition given on the definitions list on http://www.myArmoury.com/feature_properties.html: "This can easily be found by balancing the sword, lengthwise, upon your finger." For a straight symmetric weapon, this is the same as the CoG/CoM.

For a curved, or otherwise asymmetric weapon, where the CoM lies off the weapon, the PoB will be, in most cases, the point on the weapon closest to the CoM. At such a point, a line extended from the weapon to the CoM will be at a right angle to the weapon [1], and the weapon will balance, e.g., on a finger, if supported at that point. The weapon will also balance at any other point where this line is at such a right angle, even if that point is further from the CoM. (Such a point will be locally closer; it will be closer than nearby points on the weapon.) Since the weapon balances at that point, you could call it a PoB.

But "equal mass on either side of it" is wrong, for either of CoG/CoM or PoB. Consider a 10oz and a 1oz weight on a long, light stick. The CoG/CoM and PoB will be the same, since it's straight and symmetric. This will be between the two weights, so there will be 10oz on one side, and only 1oz on the other - not equal masses. The 10oz weight will be closer; perhaps this is what you meant by "equally distributed".

Perhaps it's better to just give the definition of CoM/CoG mathematically, and note the relationship with the PoB (and indentity with, in the straight case). Both definitions of PoB, on the Beginner's Glossary, and the Blade Properties list, equate CoM/CoG with PoB, which as you say isn't right in general. Otherwise, the Blade Properties list is good, and the Beginner's Glossary is wrong.

But the current definitions are mostly OK, and more probably more useful to most readers than a precise mathematical definition. (But the "equal mass" part should go.)

(I could say more about the definition of centre of percussion, on both lists. It is a common (mis-)usage of the term - perhaps this makes it worse, since it reinforces misinformation?)
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