Bowhunting Fact: KE is an extremely weak predictor of arrow penetration. This video demonstrates why Momentum, Mass and FOC are quantifiable predictors of penetration at arrow impact.
The Power of Momentum
Most bowhunters use Kinetic Energy to determine whether their arrows will have enough energy to harvest animals efficiently. That’s too bad because kinetic energy doesn’t even have a direction. It’s a measurement of the TOTAL ENERGY an arrow and broadhead have at a given moment, but it is NOT an indicator of whether your arrow and broadhead set-up has a high probability of penetrating an animal. Momentum on the other hand is the measure of a force moving in a specific direction over period of time. Momentum tells us how much energy, based on the weight and velocity of the arrow, it has available to use during impact and penetration before it stops penetrating and comes to rest. Momentum is a much better indicator of what we can expect for penetration from our arrows.
Note: Even knowing our momentum is not enough. When discussing penetration the physical characteristics of the arrows and broadheads come in to play as well.
See: Top 12 Penetration Enhancing Factors for the rest of the story.
The kinetic energy formula is: “ke = 1/2 Mass times Velocity squared” or “ke = 1/2 m • v²”
Since the velocity is squared, relatively small increases in speed bump your ke quickly. That seems to make a logical case in favor of lightweight, fast arrows right?
Here’s the problem. During flight and especially during penetration, velocity sheds its energy twice as fast as momentum does. Light fast arrows lose energy much faster than slow heavy arrows. That’s because heavier slower arrows carry more momentum.
The momentum formula is: “Momentum = mass times velocity” or “p = (m * v)”
Good Luck Stopping a Train (Or a GrizzlyStik)
The same physics apply to stopping higher grain weight arrows with a high FOC. Once in motion, they want to stay in motion.
Note: Most people measure kinetic energy at the bow, but for a more relevant number we should measure at the point of impact shouldn’t we?
Another problem is that, upon impact, the faster the object striking another is moving, the more the object being struck resists. In other words, the animal’s tissues will actually resist the penetration of a fast moving projectile more than a slower moving projectile. End result? It’s easier to penetrate animals with slower moving heavy arrows than with faster lightweight arrows.
Heavy slower moving arrows lose less energy in flight and during penetration than lightweight fast arrows. This is because of inertia. (…an object in motion tends to remain in motion.) Think of a golf ball and a ping-pong ball. They’re both about the same size but even if the ping-pong ball is launched twice as fast as the golf ball, the ping-pong ball does not have the weight to keep it in motion. It loses its energy much faster than the slower moving but heavier golf ball. Even at only ten feet away, which would you rather be hit with, a ping-pong ball moving at 300 feet per second or a gold ball moving at 150 feet per second?
When you first launch the ping-pong ball, because the formula is slanted in favor of speed, it may very well have a higher kinetic energy rating. The golf ball however retains its energy because of its higher momentum factor and is able to deliver down range where it’s needed most.
Try this simple test: Take two lightweight (300-400 grains) arrows; on one, install an 80 grain field point or, to prove a point, no point at all. Throw that arrow as far as you can. Now take the other arrow and install a 315 grain field point. Throw that arrow as far as you can. What happened? The lighter arrow, especially if you used no tip at all, wiggled and wagged, was very unstable, AND didn’t travel nearly as far as the more stable, heavier tipped arrow. Hmmmmmm…
Now, to really make a point, throw one of our full length tapered GrizzlyStik carbon arrows with a 315 grain field point on it. Quite a difference! That’s the power of momentum. That’s the advantage of forward thinking.
Case Study - Why KE doen't Measure Arrow Penetration
“Cape Buffalo with a Baseball?” – In the late 1940’s, US military trauma surgeon Dr. Frank Chamberlin researched the theory of shock waves produced by bullets traveling over 2500 fps. The term used to describe the “wave” was Hydrostatic Shock and the formula used to measure the bullets potential “wave” was Kinetic Energy (KE). Dr. Chamberlin never used the KE formula to determine bullet penetration. Instead Chamberlin determined that KE killed through the hydraulic effect “shock wave” the was produced by a supersonic projectile. Hydrostatic shock killed by trauma to body cells not by hemorrhage from bullet penetration.
Incorrectly benchmarked in bowhunting, KE has absolutely no indication of penetration or for measuring the tissue destruction protentional of projectiles traveling less than 2500 fps - none.
On the other hand, Dr. Ed Ashby’s 30 years of arrow penetration research quantified momentum, arrow mass and forward-of-center (FOC) as true predictors of penetration at arrow impact. Ashby’s research especially holds true when heavy bone is encountered. Arrows and broadheads kill through laceration (hemorrhage) created by penetration - not hydrostatic shock. The deeper an arrow penetrates, the higher the chances of lethality and animal recovery.
This video demonstrates our point. The government of South Africa requires bowhunters to produce 80 ft/lbs of KE to hunt Cape Buffalo. A major-league baseball weights 2300 grains. Using the KE formula - 2300 grains x 147 fps (100 mph) = 114 ft/lbs of KE. Using the KE logic, any pitcher that can throw 100 mph is legal to hunt Cape Buffalo with a baseball.
Ask yourself, with that high of a KE number (114 ft/lbs), how far do you think a fastball will penetrate a Cape Buffalo?