ORIGINAL: Paul L Mohr

It's not the same thing though because KE is only one part of equation. 60 ft/lbs is nothing like 60 lbs of sand, or gold. One is an energy figure the other is mass, two completely different things.

Kinetic energy is the work needed to acclerate a body of a given mass from rest to its current speed. So it's more of measurement of how something accelerates vrs how much energy it can transfer once it gets there. It has no direction, it is simply a measurement of how much work is being done.

Momentum has more to do with how an object slows down once it reaches a certain speed. It is directional in that it wants to continue in motion along the same line until something acts against it. Momentum is sort of the resistance to this force. Momentum is more simple as well as in if you double the mass or speed of an object you also double the momentum. The heavier an object is the harder it is to slow down.

So momentum is a much more imporatant figure when deciding penetration. A heavier object with more momentum also has the potentiel to transfer more energy to another object since it has more resistance to change and will want to continue in the same direction. It also retains speed for a longer distance. So a heavier object will hold more momentum over a distance than a lighter one will.

An extreme example would be which would you rather get hit with, a wiffle bat or a heavy wood one? I can swing a plastic wiffle bat much faster than I can a heavy wood one. Both probably producing the the same amount of energy when you compare rates of acceleration. However I bet the wooden bat is going to hurt a whole lot more because it won't want to slow down when it comes in contact with your body, or bounce off or deflect.

A more realistic comparison would be to compare an extremely light arrow to a fairly heavy one. Lets say a 200 grn arrow and a 800 grn arrow.

The 200 grn arrow at 370 fps would have 61 ft/lbs of energy and .328 lb/sec of momentum out of the bow.
At 40 yards the light arrow would retain 47 ft/lbs of energy and .290 lb/sec of momentum.
That is a 14 ft/lbs loss of Ke and .038 lb/sec of momentum.

Now the 800 grn arrow will produce 61 ft/lbs of Ke at 185 fps and .656 lb/sec of momentum right out of the bow.
That is twice the momentum at half the speed.
At 40 yards the heavy arrow will retain 58 ft/lbs of energy and .640 lb/sec of momentum.
For a 3 ft/lbs loss of Ke and .016 lb/sec of momentum.

So while shooting an ultra light arrow will work, it isn't the same as a heavier arrow even if they have the same Ke.

It is merely a measurement of how much energy your bow can produce, or how much work it can do.

The real question is always "How much do you need?". When is too little not enough and too much more than you need?

Paul

ORIGINAL: Paul L Mohr

It's not the same thing though because KE is only one part of equation. 60 ft/lbs is nothing like 60 lbs of sand, or gold. One is an energy figure the other is mass, two completely different things.

Kinetic energy is the work needed to acclerate a body of a given mass from rest to its current speed. So it's more of measurement of how something accelerates vrs how much energy it can transfer once it gets there. It has no direction, it is simply a measurement of how much work is being done.

Momentum has more to do with how an object slows down once it reaches a certain speed. It is directional in that it wants to continue in motion along the same line until something acts against it. Momentum is sort of the resistance to this force. Momentum is more simple as well as in if you double the mass or speed of an object you also double the momentum. The heavier an object is the harder it is to slow down.

So momentum is a much more imporatant figure when deciding penetration. A heavier object with more momentum also has the potentiel to transfer more energy to another object since it has more resistance to change and will want to continue in the same direction. It also retains speed for a longer distance. So a heavier object will hold more momentum over a distance than a lighter one will.

An extreme example would be which would you rather get hit with, a wiffle bat or a heavy wood one? I can swing a plastic wiffle bat much faster than I can a heavy wood one. Both probably producing the the same amount of energy when you compare rates of acceleration. However I bet the wooden bat is going to hurt a whole lot more because it won't want to slow down when it comes in contact with your body, or bounce off or deflect.

A more realistic comparison would be to compare an extremely light arrow to a fairly heavy one. Lets say a 200 grn arrow and a 800 grn arrow.

The 200 grn arrow at 370 fps would have 61 ft/lbs of energy and .328 lb/sec of momentum out of the bow.
At 40 yards the light arrow would retain 47 ft/lbs of energy and .290 lb/sec of momentum.
That is a 14 ft/lbs loss of Ke and .038 lb/sec of momentum.

Now the 800 grn arrow will produce 61 ft/lbs of Ke at 185 fps and .656 lb/sec of momentum right out of the bow.
That is twice the momentum at half the speed.
At 40 yards the heavy arrow will retain 58 ft/lbs of energy and .640 lb/sec of momentum.
For a 3 ft/lbs loss of Ke and .016 lb/sec of momentum.

So while shooting an ultra light arrow will work, it isn't the same as a heavier arrow even if they have the same Ke.

It is merely a measurement of how much energy your bow can produce, or how much work it can do.

The real question is always "How much do you need?". When is too little not enough and too much more than you need?

Paul

ORIGINAL: sngehl01

Clearly, your thinking process is the exact same as mine.

That's what I was thinking the whole time. 60 ft/lbs is 60 ft/lbs. It's like asking what weighs more, a pound of gold ora pound of feathers.

ORIGINAL: Paul L Mohr

I think we understand what you were looking for, and I think you probably got the answer you were looking for. However sometimes we post additional information or keep a thread running because there are other people that will read it with a similar question and can get more out of it.

And this was your quote so I was just trying to point out that it wasn't the same, at least as far as archery and penetration are concerned. Some would read that and think it was all the same, light arrow or heavy arrow. As long as it had the same Ke it would act and perform the same way.

I would like to add to my above post as well. For one I mentioned transferring energy. Honestly that means little in archery, you actually don't want to transfer any energy. You want the penetration so you want the arrow to retain the energy, not transfer it to the target. You want energy transfer with a bullet so it causes a larger wound channel.

Also a bow is basically just like a spring or an accumulator. All it does is store energy and release it. How much Ke a bow or arrow has is just an expression of how much energy it can store. If you don't change anything with the mechanics of a bow, like change the draw length or draw weight it pretty much stays at that Ke. Whether you shoot a light or heavy arrow it will still produce nearly the same Ke. It may vary a little bit, but it will normally be pretty close. The only thing that changes is speed and momentum. And the heavier arrow will penetrate better and/or hit the target harder.

The key is to find the point of diminishing returns. A heavier arrow will almost always produce more momentum. What you need to do is find out where it starts to level off with your set up. The difference between a 500 grn arrow and an 800 grn arrow might not be big enough to lose the extra speed and trajectory. But the difference between a 300 grn arrow and a 500 grn may be significant and well worth the lost speed. It's something you need to figure out for yourself though. And the OnTarget software and others are pretty good for charting this kind of information.

I have found with my set up around 400-450 grns is a good medium for me. Someone else's set up may be different.

Paul

ORIGINAL: Paul L Mohr

I think we understand what you were looking for, and I think you probably got the answer you were looking for. However sometimes we post additional information or keep a thread running because there are other people that will read it with a similar question and can get more out of it.

And this was your quote so I was just trying to point out that it wasn't the same, at least as far as archery and penetration are concerned. Some would read that and think it was all the same, light arrow or heavy arrow. As long as it had the same Ke it would act and perform the same way.

I would like to add to my above post as well. For one I mentioned transferring energy. Honestly that means little in archery, you actually don't want to transfer any energy. You want the penetration so you want the arrow to retain the energy, not transfer it to the target. You want energy transfer with a bullet so it causes a larger wound channel.

Also a bow is basically just like a spring or an accumulator. All it does is store energy and release it. How much Ke a bow or arrow has is just an expression of how much energy it can store. If you don't change anything with the mechanics of a bow, like change the draw length or draw weight it pretty much stays at that Ke. Whether you shoot a light or heavy arrow it will still produce nearly the same Ke. It may vary a little bit, but it will normally be pretty close. The only thing that changes is speed and momentum. And the heavier arrow will penetrate better and/or hit the target harder.

The key is to find the point of diminishing returns. A heavier arrow will almost always produce more momentum. What you need to do is find out where it starts to level off with your set up. The difference between a 500 grn arrow and an 800 grn arrow might not be big enough to lose the extra speed and trajectory. But the difference between a 300 grn arrow and a 500 grn may be significant and well worth the lost speed. It's something you need to figure out for yourself though. And the OnTarget software and others are pretty good for charting this kind of information.

I have found with my set up around 400-450 grns is a good medium for me. Someone else's set up may be different.

Paul

ORIGINAL: sngehl01

Clearly, your thinking process is the exact same as mine.

That's what I was thinking the whole time. 60 ft/lbs is 60 ft/lbs. It's like asking what weighs more, a pound of gold ora pound of feathers.