What is Kinetic Energy and Why does it matter?

What better time for a physics lesson than the start of the fall bowhunting seasons?

If you’ve never heard the term mentioned, you should familiarize yourself with it before you head out into the field with a bow and arrow in pursuit of wild game.

Kinetic energy.

What is it?

According to physicsclassroom.com, kinetic energy is “the energy of motion”  ‑ or, the energy possessed by an object in motion.

The amount of kinetic energy a moving object possesses depends on two factors – its weight and its speed.

The heavier an object is, and the faster it’s moving, the greater its kinetic energy.

In the bowhunting world, archers should be concerned about the amount of kinetic energy (KE) carried by their arrows as they hit game animals.

An arrow needs sufficient KE to punch through hide, tissue and possibly bone. And generally speaking, the bigger the animal, the greater an arrow’s KE needs to be to carry it through the vitals.

KE, incidentally, is measured in foot-pounds.

You can think about KE this way. Let’s say a ping-pong ball and a golf ball both traveling at 30 mph hit you in the head. They’re both about the same size traveling at the same speed, but the golf ball is heavier, and is obviously going to hurt more. Its KE is much higher than the ping-pong ball’s.

To calculate your arrow’s KE, you will need to weigh it as you’d shoot it. That is, weigh your exact hunting arrow, with the broadhead attached.

You want to come up with your arrow’s weight in grains, so adjust your scale accordingly.

Next, shoot your arrow through a chronograph to come up with its speed.

Once you have the weight in grains and the speed you can complete the following equation:

Velocity squared times weight divided by the constant 450,240.

(The constant number is derived from the conversion of grains to pounds, while factoring in the effect of gravity on mass.)

So let’s say you have an arrow that weighs 400 grains that flies at a speed of 290 feet per second.

Your equation would look like this:

290 x 290 x 400 / 450,240 = 74.72 foot-pounds of KE.

What do you need to get the job done in the field?

There are a number of charts that recommend a certain amount of KE for game animals of different size.

Generally, the recommendations are as follows:

Kinetic_Graph

 

What can you do to boost your arrow’s KE?

For starters, you want to make sure you are maximizing the available KE by tuning your bow so your arrows fly straight. A fishtailing arrow won’t fly as fast as it could. And make sure you’re using razor-sharp broadheads. The more energy it takes to push an arrow through an animal, the faster it’s going to slow down.

You can shoot a heavier broadhead. That’s going to slow down your arrow a bit, so you’ve got to repeat the KE calculation to make sure you’re getting a benefit from the switch.

You can switch to a heavier arrow shaft. It’s also going to fly slower, so be sure you recalculate your KE.

And make sure you don’t change the arrow’s spine when you switch to a heavier shaft.

(Click here for a discussion about arrow spine vs. arrow weight.)

Compound bow shooters can increase the draw weight of their bows by turning the limb bolts clockwise, provided they’re not already at the maximum weight limit.

Be sure to turn the top and bottom bolts exactly the same. That is, if you put half a turn into the top bolt, do the same to the bottom.

Recurve archers who shoot takedown bows might want to switch to limbs with a heavier draw weight.

Increasing draw weight increases arrow speed regardless of the bow.

Of course, that can make your bow difficult  – if not impossible – for you to draw, so proceed with caution.

Know your KE before you hit the woods this season. It can mean the difference between success and failure.