The Truth about Kinetic Energy and Momentum for Deer Hunting

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 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.

WHAT IS KE
We know that KE is the function of speed and mass of a moving object without direction. 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 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.

Momentum vs KE Blog Article: https://bit.ly/3ydzj2t


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