KE vs. Momentum: The Physics of Penetration
Which metric matters most for a clean, ethical kill?
Bowhunters love to debate ballistics. You'll often hear people bragging about their bow's 'Kinetic Energy' (KE), but in recent years, the focus has shifted toward 'Momentum' (p). While both are calculated from weight and speed, they represent very different physical properties. Understanding the difference is key to building an arrow that can pass through a trophy animal.
The Kinetic Energy Trap
KE is the measurement of an object's ability to do work (crushing bone). The formula is KE = 1/2mv². Because velocity is squared, fast bows with light arrows generate impressive KE numbers on paper. However, KE is easily 'spent' on friction. A light, fast arrow may have high KE but stop abruptly when it hits a rib because it lacks the 'push' to keep going.
The Power of Momentum
Momentum is the measurement of an object's resistance to stopping. The formula is p = mv. Notice that mass (weight) and velocity have equal weight here. In the messy real world of fluid and bone, momentum is a much better predictor of penetration. A heavy, slower arrow with high momentum will keep pushing through an animal, resulting in a more reliable pass-through and a better blood trail.
Calculating for Your Bow
Archery ballistics use grains for weight and fps for speed. To get KE in ft-lbs, the formula is (Grains x FPS²) / 450,240. For Momentum in slug-fps, it is (Grains x FPS) / 225,218. A 'good' setup for deer usually has at least 0.40 slug-fps of momentum. Large game like elk or buffalo should aim for 0.55 or higher.
Building the 'Adult' Arrow
To maximize penetration, move toward a 'heavy' arrow build (450-600 grains). While this slows the arrow down and makes the trajectory more arched, the gain in momentum and quietness of the bow is worth it. Combine this with a high-quality fixed-blade broadhead to ensure that none of your hard-earned momentum is wasted on mechanical parts failing.
FAQ
Is speed or weight better for penetration?
Weight is more reliable. Doubling the weight doubles the momentum, whereas doubling speed only doubles momentum but quadruples KE. In archery, you almost always run out of momentum before you run out of KE.
What is a 'pass-through' and why does it matter?
A pass-through occurs when the arrow exits the other side of the animal. This creates two holes, resulting in faster blood loss and a much easier tracking job for the hunter.
Do mechanical broadheads require more momentum?
Yes. Mechanical heads use energy to 'deploy' their blades. If you have a low-momentum setup, a mechanical head can use up too much energy opening, preventing the arrow from reaching vital organs.