Center of percussion

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The center of percussion is the point on a bat, racquet, sword or other long thin object where a perpendicular impact will produce translational and rotational forces which perfectly cancel each other out at some given pivot point. The center of percussion may or may not be the "sweet spot" depending on the pivot point chosen.

1 Further explanation
2 Calculating the center of percussion
3 Relation to the sweet spot
4 Application to swordmaking
5 See also

[edit] Further explanation

Imagine a beam is suspended from a wire by a U-bolt so that it can move freely along the wire. A blow to the beam below the center of gravity will cause the beam to spin around the CoG and also cause the CoG to move away from the blow. If the blow falls above the CoP the movement of the CoG will cause the U-bolt to move away from the blow since the effect of the translational acceleration will outweigh the effect of the rotational acceleration. If the blow falls below the CoP the opposite will occur, rotational acceleration will outweigh translational acceleration and the U-bolt will move towards the blow. Only if the blow falls exactly on the CoP will the two cancel out to produce no net movement of the U-bolt.

[edit] Calculating the center of percussion

In a free beam of uniform density, a force F applied at a right angle at a distance b from the center of gravity (CoG) will result in the CoG moving at a velocity V according to the relation:

$F=M\frac{dV}{dt}$

Where M is the mass of the beam. Similarly the torque exerted will be as per the relation:

$Fb=I\frac{d\omega}{dt}$

Where I is the moment of inertia around the CoG and $ω$ is the angular velocity.

For any point P on the opposite side of the CoG from the point of impact:

$v = V - A ω$

Where A is the distance of P from the CoG and v is the velocity of point P. Hence:

$\frac{dv}{dt}=\left(\frac{1}{M}-\frac{Ab}{I}\right)F$

v is then given by:

$v=\left(\frac{1}{M}-\frac{Ab}{I}\right)\int F dt$

The axis of rotation is situated where $v = 0$ and the corresponding center of percussion is at distance b from the CoG where:

$b=\frac{I}{AM}$

and for a beam of length L the moment of inertia around the CoG is:

$I=\frac{ML^2}{12}$ (see moment of inertia for proof)

[edit] Relation to the sweet spot

The sweet spot on a baseball bat is generally defined as the point at which the impact feels best to the batter (it is also occasionally defined as the point at which the maximum velocity is imparted to the ball, but this may not be the same point).

Although it has long been believed the center of percussion and the sweet spot are the same, recent practical observations have indicated that the point many batters feel is "sweetest" corresponds to a pivot point not in the handle of the bat but beyond the end of the bat. The reality of, and explanation for, this anomaly are currently areas of active research.

[edit] Application to swordmaking

The center of percussion, or sweet spot, of a sword is the point on the blade where cutting is most effective. It is also the division between the weak and middle sections of the blade.

Like any solid object, a sword vibrates when impacted (such as during cutting). In a sword, such vibrational waves are typically almost imperceptible. Every wave expressed by a solid object has rotational nodes where the wave reverses at either end of the object. On a properly-balanced sword, one node is in the tang of the sword (inside the hilt), ideally directly under the primary hand. The other is the center of percussion. On such a “harmonically balanced” sword, this means that a solid blow can be delivered without causing discomfort in the hands. The center of percussion of a sword is related to its center of balance, and both can be moved by employing a heavier pommel or changing the mass distribution of the blade.

So-called “blade harmonics” are a commonly misunderstood concept, especially by sword enthusiasts and their internet communities. The common belief is that a sword must be “harmonically balanced” in order to cut properly, because the vibrations would otherwise interrupt the line and power of the cut. As explained above, this proposition is false: the vibrations caused by a sword cut are almost unnoticeable except as a mild stinging to the hands even in blades that lack this quality. It has also been demonstrated that the object the sword cuts through serves to further reduce the intensity of any vibration, making it even less noticeable.

Many experts speculate that harmonic balance is merely a byproduct of proper construction and balancing, rather than an intentional quality added to weapons. Unfortunately, some sword vendors advertise "secret techniques" of harmonic balancing in an attempt to "prove" the superiority of their products. This only serves to amplify the false impressions of the value of harmonic balance by seeming to lend them legitimacy.

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