momentum question???

[ozy clint]

was just reading some of doc ashbys' reports. i want to know how to determine what proportion of a given arrow momentum is derived from it's mass? eg. i want to compare a 700gr arrow @ 150fps to a 390gr arrow at 300fps. the doc goes on to say that the heavy arrow will out penetrate the lighter one, even though the lighter one has more momentum. this is because the light arrow encounters more resitance, because it's travelling faster. the heavy arrow achieved more penetration because it gained a greater percentage of it's momentum from it's mass than it did from it's velocity. how fast would the light arrow need to going to achieve the same penetration? i guess what i'm trying to work out is how to compare the penetration of arrows of unequal mass, unequal velocity and unequal momentum. confused yet? i bet a mate  that i could outpenetrate his compound with my recurve. he is shooting a 450gr arrow at 290fps from his 70# wheelbow ( that speed is worked out from his bows speed rating)accurate?? don't know. i'm shooting a 780gr arrow from my 58" 69# bob lee drawn to 28" speed unknown. maybe someone has an idea what fps that arrow may be doing. which of us will be drinking the free beer?

[Soilarch]

Unless he makes a poor choice in broadheads...or doesn't have his arrow/bow tuned, I'd say you're buying.

I can't help with the whole momentum and velocity thing.  Figuring the numbers for those things are easy enough...but add in the part about the higher rate of momentum loss from a higher velocity and your outside of my scope.

BUT HERE'S WHY I SAY YOU MIGHT BE BUYING:
A bow can never "put out" more energy than you "put in".  He's putting an extra pound of energy "in".  Now, there's more "lazy" weight in a compound's limbs, cams, and string/cables than a tradbow's lighter limbs and singular string.

Heck, now that I say that it might be a dead race.

I'd pray he doesn't have a perfect tune and can't sharpen a BH worth a hill of beans, then you know you've got a free drink coming.

When you coming State's side, and what are you wanting to pester with a pointy stick?     :D  


P.S. I would feel safer betting that he is NOT shooting 290fps. (Even if calculations are spot-on) Those "posted" IBO speeds are on just this side of worthless. How many strands in the string? Did the arrow have any fletching? Did they 'hardwall' the bow? (Same as forcing the bow to stack.)  If the bow is made in two different let-off percentages they will always post the speeds of the lower let-off cam because they're faster...and most every hunter buys the higher let-off cams.  Also, they don't post the 'average' speed, usually they pick the top-dog and run with it.  Also, he probably ISN'T pulling 70# if his limbs are simply marked 70#.  My compound is marked 70# but actually pulls 74#.  Obviously this may work for or against you  

[O.L. Adcock]

Ozy, When reading Doc Ashby's studies you must keep in context that he is looking at optimizing "A" bow....The bow you have in your hand...When you start trying to compare this arrow to that from different bows it's talking apples to oranges.

Your arrows, the 700 and the 390...those are out of different bows with vastly different KE's with the 390 having almost double..But I'll put my money on the 700 set up as the Doc suggest compared to the 390 that isn't!    Resistance to penetration goes up with the square of the velocity so the only conclusion is his bow IF he was shooting the same speed you are (his arrow would be way heavier to do so), he'd kick your butt, but the way you state it, I'd bet on yours as long as you picked a decent broadhead. Here is how momentum works out of any bow, trad or compound...O.L.