Whether you’ve been bowhunting for a day or a decade you’ve likely heard the longwinded arguments about Kinetic Energy. Rednecks everywhere that probably still struggle with long division can calculate and tell you the kinetic energy of their bow, arrow and broadhead set up. Well I’m here to shatter dreams and tell you that Kinetic Energy doesn’t really matter that much. Ok, it matters but it’s not the whole story. If you’re looking for bone splitting knockdown power, you need to consider momentum.
We have a few things at play here. I’m not going to do a ton of math because I think just some simple explanation will do and if you don’t believe me then go ahead and get your calculator and go nuts. We’ll start with a few definitions:
Kinetic Energy: energy that a body possesses by virtue of being in motion.
Momentum: the quantity of motion of a moving body, measured as a product of its mass and velocity.
If your like me that doesn’t tell you a whole lot. So here are the formulas that go along with them. The main difference is momentum takes into account direction not just energy.
KE (of an arrow)= (weight in grains X speed in feet per second squared)/450800
Momentum (of an arrow) = (weight in grains X speed)/225400
Again, I don’t know if that tells you much but wanted you to have all of that as a reference in case you don’t like my examples and explanation that follow. Now you can crunch some numbers or check out this calculator to crunch some numbers for you.
To cut to the chase here and why momentum is so important,here is a real life example. If you’re 20 feet away from me, would you rather me throw a ping pong ball at you or a golf ball? I’ll throw them with the exact same effort. You likely said ping pong ball. The reason, the ping pong ball will have less momentum. The ping pong ball will go a lot faster the first few feet but it will shed energy a lot faster than the golf ball. While the golf ball won’t move as fast, it will retain a lot more energy until it hits you.
Also, we need to look at what a heavier arrows does to your bow. There’s no doubt that if you shoot a very light arrow then shoot a very heavy arrow there is an obvious difference in sound and vibration. The reason is your bow is a machine and it works better with heavier arrows. The sound and vibration is wasted energy. With a heavier arrow your bow will be able to more efficiently use it’s stored or potential energy to propel the arrow, resulting in less noise, less vibration and more energy making it to the arrow.
So now what happens down range. We touched on this with the ping pong ball example but if we look to Isaac Newton and his second law of motion we see that Force = Mass X Acceleration. Sir Newton also tells us that an object in motion tends to stay in motion unless acted upon by another force. So what we have until our arrow encounters its target is air resistance, and this will be basically very similar between a heavier and lighter arrow. What this equation tells us is that heavier objects take more resistance to slow them down. If the arrow has more mass it will take more of that force with it down range. That is what we want right? We want to have the most force when our arrow meets it’s target. We aren’t just trying to get an arrow to stick into a hay bale we want to blow through a living animal and take out it’s vitals.
What this all boils down to is a heavier arrow, while it may move a little slower at first will make your bow more efficient with less vibration and noise, the arrow will have more energy as it exits the bow and will retain more of that energy and speed down range. Go ahead and go nuts with the calculations, you’ll notice that as you increase arrow mass, momentum continues to rise sharply while kinetic energy levels off pretty quickly.
Yes there is an arrow that is too heavy I’m not going to start shooting arrows made of lead, but I’d encourage you to take a look at GrizzlyStik arrows. This is not some long winded advertisement or sponsored ad because well they sponsor no one, so I won’t have to do a bunch of convincing there. They are the only carbon arrow company talking about momentum and it’s importance when it comes to being a consistently deadly bowhunter. They’ve posted the proof all throughout their website with pictures of massive game falling to their heavy arrow and broadhead combinations. They also have the 650 Challenge, (650 referring to the weight in grains of the arrows and braodhead set up), where if you buy two test arrows then turn around and buy a half dozen they take the cost of the two arrows off the half dozen, so you’ll end up with 8 arrows for the price of 6.