What's the difference between momentum and kinetic energy?
When it comes to bowhunting, kinetic energy gets all the love. You can find scads of articles extolling its virtues and charts describing its professed impact.Momentum, by comparison, is the genius, recluse cousin few people talk about, but who has a cult following. It’s punk rock before that brand of music became mainstream.In another blog entry, we explored how kinetic energy relates to bowhunting. Kinetic energy is described as the energy of motion. The faster and heavier an object is, the more KE it carries.Lots of hunters swear by KE, and they do whatever they can to maximize it with their bowhunting rigs.But there are others who could care less about KE, and instead focus on boosting their arrow’s momentum. Both account for the weight and speed of a moving object, but they are not the same.One of the best explanations I’ve heard about how to differentiate KE from momentum comes from the Quality Deer Management Association, which is a true believer in momentum.QDMA explains that, with KE, speed is emphasized over weight. An arrow’s KE is calculated by the equation: velocity squared times weight divided by the constant 450,240. That results in a unit of energy in foot-pounds. That’s the amount of energy needed to exert a 1-pound force for a distance of 1 foot.Momentum, QDMA says, emphasizes weight over speed. An arrow’s momentum is calculated by taking weight times velocity and dividing by the constant 225,400. (The constant accounts for the arrow’s weight in grains and factors in the pull of gravity.) This gives you a unit of force measured in slug feet per second. A slug is a unit of mass that accelerates by 1 foot per second per second when acted upon by a 1-pound force.So my Easton FMJ that weighs 485 grains and flies at 292 feet per second has .63 slug ft./sec. worth of momentum.(And therein lies the likely reason momentum takes a backseat to KE. Who knows what .63 slug ft./sec. means other than a physicist?)Momentum is a measurement of the force of the forward movement of an object. KE is a measurement of the energy a moving object possesses, but it has no direction.Momentum advocates say KE tells you how hard an arrow will hit, but that has nothing to do with penetration. Momentum is what allows an arrow to blast through tissue, hide, bone, etc.QDMA suggests that momentum is particularly more important than KE when archers are working with low-poundage bows – say, below 60 pounds. Archers can increase their KE by switching to lighter arrows, which will boost the arrow’s speed. But that lighter arrow isn’t going to have the same momentum as a heavier one, and so it takes less resistance to stop its forward progress.Generally speaking, it’s often recommended that bowhunters using compounds shoot arrows that weigh 6-8 grains per pound of draw weight. That would require a hunter using a 60-pound bow to shoot an arrow weighing 360-480 grains. The bigger the game, the heavier you’ll want to go.(If you really want to push the momentum envelope, check out the super heavy shafts that weigh more than 9 grains per inch for a 400-spine arrow. The Easton FMJ weighs 10.2 GPI ; Gold Tip Kinetic Hunter weighs 9.5 GPI; and Carbon Express PileDriver weighs 10.4 GPI.)To keep your arrow’s momentum as high as possible when shooting compounds that draw less than 60 pounds, stick to the high end of the 6-8-grains-per-pound weight range. So if you’re shooting a 40-pound bow, go for an arrow that’s in the neighborhood of 320 grains.Traditional bowhunters tend to stick to an average of about 10 grains of arrow weight per pound of draw weight in the interest of generating lethal momentum from a bow that produces relatively slow arrow speeds.(Of course, arrow selection always starts with matching the spine to your bowhunting setup. Don't deviate from that just to gain arrow weight.)As QDMA notes, no one apparently has done any tests to devise a chart that suggests how much arrow momentum is needed to punch through various game animals. So there don’t seem to be any definitive recommendations for momentum.The good news is, modern bow and broadhead designs are making bowhunters more effective at achieving the lethality we owe the animals we pursue. The discussion continues, and our understanding will too.