gzg38b

Nope. DISTANCE on the string is power stroke. That's why powerstroke is measured in inches, not seconds. Time has nothing to do with it.

TFOX

Read my thread again,you will see I said EXACTLY that in my second paragraph.

bigbulls

So you guys are saying that the amount of time an object spends under acceleration has nothing to do with its speed/ velocity?

98Redline

bigbulls, you are mixing apples and oranges.

If acceleration is held constant, then time under acceleration will effect the final velocity, however on a bow as gzg38b pointed out, the Potential Energy (area under the draw force curve) is a constant. It wouldn't matter if it took 10 seconds or .01 seconds for the arrow to get off the string, it will only have as much energy leaving as you put into the bow on the draw - the efficency losses (friction, vibration, sound). The only way to increase KE is by increasing the area under the draw force curve (more draw length or weight) or by increasing the efficency of the bow (remove weight from the string, reduce friction etc...) however these changes are all related to the bow and would then cause a change in both the heavier and lighter arrows.

bigbulls

So now explain to all of us how"X" rifle will fire a 150 grain bullet faster than it will fire a 180 grain bullet but the 180 grain bullet will have more KE and momentum. Ifboth bullets are being pushed by 60,000 psi of chamber pressure how exactly does the pressure pushing the heavier bullet become more efficient than the pressure pushing the lighter bullet?

Regardless of what we want to call itthe heavier arrow will have more KE and momentum. It may not be a whole bunch but it will be there.

98Redline

The reason for a given powder charge that a 180gr bullet will have more KE than a 150gr bullet is straight physics.

On any firearm and load, the powder charge is not completely burned and fully expanded before the bullet leaves the muzzle (muzzle flash and report). Any gas expansion or powder burn that occurs after the bullet passes the muzzle is lost energy.

Given that, the 180gr bullet has more mass its acceleration will be slower than the 150gr bullet. This will result in more time for the powder charge to burn. This extra time between the 180 and 150 muzzle exit times allows more powder to burn and expand behind the 180gr bullet than the 150. (thus more energy is available to push the bullet), also resulting in higher sustained chamber pressures and more area under the pressure vs bullet displacement curve than the 150gr bullet. Because of this your 180gr bullet has more KE leaving the muzzle than the 150gr bullet.

The difference between the bow and the gun is the bow expends all of it's potential energy into the arrow. The gun has excess energy that is wasted at the muzzle. To make an apples to apples comparison between the bow and gun, you would have to reduce the powder charge so that the powder was completely burned and the gas had completely expanded by the time the bullet left the muzzle. In that condition (fully burned powder = constant KE produced) both the 150 and 180gr bullet would have the same KE at the muzzle.

bow_hunter44

Or in other words, all of the PE in the bullet is stored (as chemical energy)in the powder. If ALL of it is converted to KE then at the instant the bullet leaves the barrel (assuming all of the powder is burned and gas expansion is complete, of course) PE = KE! This would then be analogous to the bow and arrow.

gators37

We tried a little experiment today. We shot 3 arrows with different weights, same bow same draw weight and DL. As for the results, thegrains/velocity were 358/256, 500/221, and 1242/144. The KEs were approximately 52.11, 54.24, 57.20 respectfully. Although the PE "for the bow" should be considered as constant, "my bow" appears to be moreefficient intransfering itsPE to heavier arrows. This also indicates that there may be significient losses in the transfer from thePE to KE process dependent on the arrow weight. (neglecting any effects do to the spine for now).
Could have fooled me.

bow_hunter44

I must say that I am a little surprised as well. However, after thinking about it a little... Some of the energy transferred from the bow to the arrow is lost at the arrow "snakes" away from the bow string. It stands to reason thata heavier arrow will have less oscillation than a lighter arrow. So I suppose it does make sense that a heavier arrow will have a more effecient transfer of energy than a light arrow....

TFOX

Heavy arrows absorb more of the bows pe than does a light arrow.The energy gets waisted,in lamens terms, when using light arrows.



We were talking about power stroke as being the distance the string travels and that gets translated as time on the string,kinda like when ke gets referred to as potential penetration.