bigcountry

Exactly, because wieght and momentum is already factored into BC. Not drag coef, or friction coef, but BC only. Maybe we are saying the same thing.

TFOX

No it won't that is why the light arrow will lose more speed when shot from the same bow but it won't lose as much when shot from your bow as it does when shot from his bow and then compared to your bow with a heavy arrow.


You must compare apples to apples,shoota light arrow from your bow and you will see there isn't nearly as much difference in the 2 arrowswhen shot from the same bow.Especially at 23 yards.

Arthur P

There will be minimal difference in KE but a significant difference in momentum between a light and heavy arrow shot from the same bow! Even though the difference in KE will only be a couple of Ft Lbs, a 700 grain arrow will have approximately 30% more momentum than a 350 grain arrow when shot from the same bow.

edit: LOL! Shouldn't go messing around with all this arithmetic and stuff before my 2nd cup of coffee. Had to make a correction...[8D]

Interesting note though... Say the bow you're shooting spits out your arrows with 70 ft lbs of energy. You decide you want to try and get the 350 gn arrow's momentum to match the momentum of that 700 gn arrow. You have to drive that 350 grain arrow to 430 fps and 143 ft lbs of energy to get there.

Roskoe

Mark - you know I thought about this last night, while sipping on a glass of very old Scottish whiskey. Started playing the devil's advocate against my own logic . . . . .

Suppose is I go out to my lathe and produce a rifle bullet made out of aluminum - to the exact shape and diameter of a .30 caliber 220 grain Sierra Matchking. Let's say it only weighs 125 grains. And I also produce an identical shaped bullet out of machineable tungsten - and this bullet weighs 290 grains. Both bullets have exactly the same ballistic coefficient, right? If they were launched at exactly the same velocity, I can't see any way the aluminum bullet is going to fly as far as the tungsten bullet. Kinda like steel shot opposed to lead shot. But if I plug the numbers into my Exbal or Infinity program, they come out the same. Now I'm kinda confused . . . .

Dave_G

Roskoe,

In your example of the tungsten versus aluminum bullet, they'll have the exact same shape, as you cited, but they won't have the same ballistic coefficient.

The reason is because the ballistic coefficient of a bullet is dictated by the bullet's sectional density, and the sectional density is the ratio of the bullet's diameter to its weight.

So the tungsten bullet will have the identical shape as the aluminum bullet, but will have a higher ballistic coefficient due to the higher sectional density.

TFOX

Arthur,do you not agree that a 370 grain arrow has more momentum shot at 260 fps than it does when shot at 230 fps.Therefore it will retain more speed downrange than the 370 grain arrow shot at 230 fps.


This means the 260 fps arrow willlose less speed at 20 yards than the one that is shot at 230 fps.


I agree with the momentum being the reason for a difference but you still aren't comparing apples to apples.


1 more thing,I can change the speed of my arrows just by shooting through a different spot in the chrony,up to 4 fps if I remember correctly but my chrono bit the dust so I can't say for sure.

bigcountry

Nope, they will have totally differnt BC's but the same drag coef. Two diffferne things. One has much more momentum than the other.

drhntr178

Heavier objects have more momentum, thus they are harder to slow down.

Example: Go outside and throw a baseball as hard as you can. Then throw a wiffle ball as hard as you can. See which one slows down quicker. The heavy one or the light one.

TFOX

Did you read any of the replies? That has been stated,just more to it in this discussion,actually,these discussions.

Arthur P

True, the 370 gn arrow at 260 fps will have more slightly momentum. However, it will lose a greater percentage of it's speed, even at 20 yards, than the same arrow at 230 fps. You increased the speed BUT, wind resistance (drag) increases by the square of velocity.

The 260 fps arrow will still arrive at the target with more energy than the 230 fps arrow had at the bow. The ballistics table shows the 230 gn arrow starting out at 230 fps, 43 ft lbs of energy and arriving at 20 yards doing 219 fps and 39 ft lbs. The 260 fps arrow: 260 fps, 55 ft lbs and 248 fps with 51 ft lbs at 20 yards.

That's a loss of 11 fps and 4 ft lbs for the 230 fps arrow. 12 fps and 4 ft lbs for the 260 fps arrow. The differential in energy loss between the two arrows is too slight to be worth considering at 20 yards but it is measurable... if you want to run the calculator out to that many decimal places.

Contrast: I ran a 570 grain arrow at 185 fps to match the energy of the 370 gn arrow at 230 fps and checked the numbers at 40 yards. Starting at 185 fps with 43 ft lbs, it hits the 40 yard target at 175 fps and 39 ft lbs.

That arrow lost 10 fps and 4 ft lbs. It yields roughly the same numbers at 40 yards as the 370 gn arrow put up at 20 yards. That's why it's better to get your momentum from mass than from speed.

(By the way, that 570 gn arrow at 185 fps is almost exactly the arrow I use and speed I get from my recurve.)

Just for grins, the 40 yard numbers for the 370 gn arrow: 230 fps is down to 209 fps, from 43 ft lbs to 36. Loss of 21 fps and 7 ft lbs. 260 fps arrow is down to 236 fps, 55 ft lbs is down to 46. Loss of 24 fps and 9 ft lbs. The 260 still arrives at target with more KE than the 230 fps started out with but it still lost a higher percentage of it's speed and energy.