Huh? Absolute understanding of this measurement unit undoubtedly requires some background in Newtonian physics, which is a tad beyond the comprehension level of this bowhunter! Let’s just say that momentum is a measurement of the concentrated force of an object that is moving in a specific direction at a specific point in time. The result is expressed in slug feet-per-second (slug fps), and herein lies the problem that undoubtedly limits the practical application of momentum by bowhunters – what in hell is a “slug?” A slug is a unit of mass that accelerates by 1 foot per second per second when acted upon by 1 pound of force. Momentum in terms that may (or may not) be useful to bowhunters is calculated by multiplying the weight of the arrow in grains by the arrow speed in feet-per-second, and then dividing by the constant 225,400. The formula for momentum places much greater emphasis on the mass (or weight) of the arrow. As can be seen in the formula above, velocity has a much greater influence on KE than mass. Momentum advocates argue that KE indicates how hard an arrow will hit, but actually has little to do with arrow penetration. Another way to understand this concept is to describe momentum as the resistance force required to stop the arrow as it passes through the hide, tissue, body fluids and bone of an animal. Persistence is the force that drives the arrow into an animal, which explains why “P” is the symbol for momentum. While KE is a measurement of the energy of a moving object without regard to direction, momentum is a measurement of the persistence force of the forward movement of that object. The result is expressed in foot-pounds of energy (fpe), or the energy required to exert 1 pound of force for a distance of 1 foot. The expression of KE that is most useful for bowhunters multiplies the weight of the arrow in grains by the square of the arrow speed in feet-per-second, and then divides the product by the constant 450,800. The basic formula for calculating KE is 1/2 the mass of the moving object multiplied by the square of its velocity. The greater the efficiency of the bow, the greater the percentage of energy transferred to the arrow.Īlthough some is lost in conversion, most of the energy transferred from the bow and string to the arrow is used to propel it forward, giving the arrow kinetic energy (KE) during its flight. When the string is released most of this stored energy is transferred to the arrow, while some is converted into noise and vibration, and some is consumed by friction in the moving components of the bow.
#Bear lights out arrow grains full
At full draw, this “potential” energy is stored in the bowstring and flexed bow limbs. When the bowstring is pulled and the bow limbs flex, energy is created by the archer’s work. Work must be performed on the arrow to give it energy, and this work is performed by the archer. An arrow at rest in a quiver has no energy. We know from physics that kinetic energy is a function of the speed and mass of a moving object. Let’s start with a basic review of kinetic energy and momentum as they relate to archery equipment.
Understanding how kinetic energy and momentum affect downrange performance will help to make your bow and the arrows it launches more deadly on the game that you hunt.
“persistence force”) is often overlooked, and unless you’re also a physicist, more difficult for most hunters to relate to lethality. Momentum is another important factor in the lethality equation, particularly if you’re a bowhunter. It’s generally understood that the greater the kinetic energy, the greater the killing power of the bullet or arrow. If you’re a hunter, chances are good that you know something about kinetic energy.
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