A common misconception in modern archery is about the importance of momentum.
But first lets take a step back and see what momentum actually is ...
Momentum describes movement in a certain direction, with a certain mass behind it. To get the momentum of a given object, we multiply its mass in kg with its velocity in m/s.
So, now that that is out of the way, let me explain why I think the importance of momentum is highly overestimated. For this I will compare some projectiles with equal momentum and different KE, and after that I will try to explain, what I think is the reason for this overestimation.
Lets start by comparing objects with a momentum of 5 kg m/s.
A 100 g stone moving at 50 m/s (~112 mp/h or 180 km/h) with a KE of 125 Joule
A 1 kg stone moving at 5 m/s (11.2 mp/h or 18 km/h) with a KE of 12.5 Joule
A 5 kg stone moving at 1 m/s (2.2 mp/h or 3.6 km/h) with a KE of 2.5
Joule
Here another example I stole from a thread, that really explains the issue indepth:
And as already pointed out you could ask yourself the following; What is the more dangerous to stand in the way of:
A 20 ton truck moving towards you at 0.1 m/s? (Momentum= 2000, KE= 1 Joules)
or
*A 20 kg 1 sq/m metal plate moving towards you at 100 m/s ? ( Momentum= 2000, KE= 1,000,000 Joules) *
While momentum just describes mass in motion KE tells us how much energy the object has.
Now, as to why I think momentum is getting as much hype as it is:
A bow transfers more energy into heavier arrows than lighter arrows. When the potential energy of a bow is transfered into KE to move the limbs of the bow, and the arrow, a heavier arrow gets a higher share of that potential energy.
A heavier, thicker arrow is more likely to withstand the impact and actually transfer the energy to the target, instead of breaking and losing most of the energy.
Momentum actually matters when shooting through fluids, just not when shooting elastic materials. A projectile doesn't 'drag' through a breastplate. So an arrow with higher momentum loses less energy in-flight to air resistance. (It loses more energy for different reasons though.)
While the maximum KE a bow can generate is fixed, you can tinker around with momentum. A bow potential energy depends on its draw weight, draw distance, string weight and other factors, but is practically unchangeable. Changing the weight of the projectile allows to easily change the momentum though.
I hope this cleared some stuff surrounding momentum up, and mentioned enough about arrow weight, for you to know why war arrows are heavier than those used for hunting, (its because the strain of impacting steel is substantially higher than impacting skin) and showed why momentum should be essentially meaningless for archery in CoE.
(It might be important for knocking people down though, so stay tuned, we aren't done with momentum just yet.)
For people who want to read more on the topic Link
Back to the collection thread
