As bowhunters, we have one goal. A well executed shot & perfectly placed arrow providing a clean passthrough. I want you to read that last line once more and then understand Kinetic Energy has less to do with our goal than what we've been fed. IF YOU ONLY RETAIN ONE THOUGHT FROM THIS ARTICLE, LET IT BE THIS.....ARROWS KILL THROUGH BLOOD LOSS WHICH IS OBTAINED WITH A BROADHEAD AND PENETRATION...WE NEED TRAUMA!!!
As bowhunters we've bought into the concept of high KE setups being better, but the truth of the matter is KE isn't the driving force behind our goal as bowhunters. Put a firearm in your hands and that statement flips 180 degrees.
Let's face it, there's a ton of buzz around hunting arrow builds these days. 20 years ago folks were buzzing around speed, now terms like FOC, KE, P, and total arrow weight dominate the arrow lingo. There's constant jabber about kinetic energy of your arrow but not nearly as much talk about momentum (P). At least until you talk with someone who understands physics or a bowhunter who prioritizes down range energy and penetration.
You certainly don't need a PhD to understand any of this information. You do need to do some work and acquaint yourself with some basic physics and math. Don't worry we've done the homework and we will walk you through it. To keep our points and information simple, below are a few helpful tips before we dive into Kinetic Energy, Momentum, and which you should hold as a priority as an archery hunter.
- Mass is the quantity of matter in an object. It's vital to understand that mass is not the same as weight. For example, if you took a 10lb dumbbell to the moon it weighs far less than it does on earth due to gravity. Mass is also a measure of a body's inertia, the resistance of acceleration when a net force is applied (this also is key when an opposite force is applied).
- Velocity is the rate of change of an object's position with respect to frame of reference and function of time. With that said, to keep it simple, velocity is speed and direction at a specific moment in time.
WHAT IS KE
We know that KE is the function of speed and mass of a moving object without direction, a scalar quantity. In the bowhunting world an arrow at rest has zero energy. Work, being directional force over a period of time, must be done on the arrow by your bow. Seems obvious. PE or Potential Energy stored in your bow and string is transferred to your arrow launching it forward and giving it Kinetic Energy while in flight.
To calculate Kinetic Energy we can use the following basic formula.
KE = .5Mass x Velocity².
For bowhunters, we can take the total weight in grains of our arrow as Mass. Velocity can be speed of the arrow in feet per second. The final equation to calculate your arrow's KE will be
KE = (Mass)x(Velocity x Velocity)/450,800
This expresses KE in Foot-Pounds of Energy or the amount of energy needed to move 1lb, 1ft of distance. Keep in mind, there is still no direction with KE, remember KE is a scalar quantity, and understanding this should force us to realize KE is not doing the killing with an archery setup. It simply tells us how hard the arrow will hit our target, but it becomes more into play when bowhunters choose to shoot mechanical broadheads.
WHAT IS MOMENTUM
You probably heard the saying "An object in motion stays in motion, until...."? If you have that understood, then you understand the concept of momentum. In bowhunting terms Momentum is simple, persistent force with direction or how much opposite force/resistance it will take to stop our arrow. Mass of your arrow multiplied by the arrow's velocity equals momentum. So we can note this as:
P = Mass x Velocity
For bowhunters, we can take the total weight in grains of our arrow as Mass. Velocity can be speed of the arrow in feet per second. The final equation to calculate your arrow's P will be in slugs FPS and can be noted as:
P = MV/225,400
Momentum is a vector quantity, so it has both magnitude and direction. Because Momentum has a direction, it can be used to predict the resulting direction and speed of motion of objects after they collide. For a bowhunters calculation, 1 slug is simply a unit of mass moving 1 foot per second when 1 pound of force in direction is enacted upon it.
All this sounds complicated but it boils down to this simple state. Our goals are passthrough shots, momentum being the resistance to a change in velocity is critical and THE most important aid in bowhunters achieving that goal.
Without going down an additional worm hole and making this more complicated, we just want to mention that when attempting to calculate P, there is an additional variable to rotating objects that is often left out...ROTATIONAL MASS. Without a physics lecture and additional formulas the concept is relatively simple, the higher the revolutions per minute, RPM, of your shaft the better off you are with all other variables being equal.
KE OR P: WHICH IS KING?
So with a basic understanding of physics, the real question here is...From a whitetail bowhunter's prospective, should we be paying attention to KE or P? For a deeper dive on KE and P directly related to whitetails check out this article. Quite frankly blood loss and trauma is what kills whitetails with an archery setup, let's not forget that. Broadhead penetration is what delivers blood loss. Pretty simple. Momentum delivers penetration more so than Kinetic Energy.
In addition to that point, calculating momentum or even KE through a chronograph is giving you inaccurate information! More accurate and real life scenario numbers can only be obtained by using velocity variables at distance, 20/30/40 yards and beyond. I say this because rarely will you ever send an arrow towards game at such near distances that replicate a chronograph. The average shot on a whitetail is under 30 yards, not a few feet. To gather valid, tangible information that actually depicts a real life hunting scenario where your arrow impacts an animal at distance, you need a radar. We also understand that having access to a radar isn't in everyone's back pocket, but there are several quality pieces of content you can reflect on to give you an idea of velocity deprivation at distance.
At the end of the day, penetration is what we are all after. Whether you achieve that through mass or velocity is really up to you. There are valid arguments to both sides but something we can all agree on is this....SHOOT THE SETUP YOU HAVE THE MOST CONFIDENCE IN! Archery hunting is and always will be a personal progression and journey, if you can expedite the learning curve from our experiences then we all win.
Author: Chad Sylvester, Exodus Co-Founder/Owner