The "preconcieved notion" was established when KE in archery was defined as KE=1/2mv^2 (the v being the velocity of the arrow directed at the target), I haven't re-written anything. It's hard for me to believe that you do not understand that KE in archery is defined as only the energy moving in the direction of the target and specifically exludes energy not possesing that direction as a property. Unlike the kinetic energy of a tuning fork It's very definition includes direction and excludes energy not directed towards the target! To me it's like you are denying the obvious. But I've explained as best I can, I'll accept that we disagree regarding this matter. I believe however it is this very problem of not realizing that ke in archery is not a general case but rather is uniquely defined and as a result of that definition is a vector like momentum. Analogies to tuning forks is the type of argument that adds to the confusion.

editing... I think you are looking at KE from the perspective that it can not have direction but that of course would not be correct even from a pure physics definition. KE as energy of motion simply does not have to be focused in one direction but it most certainly can be. The kinetic energy in a fly wheel or a tuning fork are examples that do not exhibit a general direction but an arrow in motion without question has direction. The arrow carries the energy therefore its energy has direction.

Dagnab it! It's so painfully obvious to me and I simply cannot understand why you won't see it!!! It's getting frustrating, but I'll try to get it across, one more time.

Let's review. KE is nothing more than the CAPACITY TO DO WORK. Period.

Without momentum telling KE what direction and over how long a period of time to do that work, it'd just sit there. When you direct an arrow at the target, you are telling momentum which way you want it to tell KE to go do it's work. It IS MOMENTUM THAT FOCUSES KE IN A GIVEN DIRECTION.

A light arrow is easier to deflect and have it's direction of flight changed than a heavy arrow. Why is that? Because it has less momentum holding it on a given direction.

The KE in a tuning fork is not focused in a given direction. Very good. You strike it and it's energy is turned into vibration, noise and heat. So, let's explore the old tuning fork. You strike it to impart KE to the thing, then it just sits there vibrating and humming until all it's energy is expended. That's because it's got no momentum. It will never move across the room on it's. You can strike it and put KE into it, then throw it. But then you've also imparted another level of KE into the tuning fork. Now, that's a whole lot more KE than it had before.

Let's take two identical tuning forks. We'll strike one but not the other and throw them at a target across the room. According to your way of thinking, the humming tuning fork, when thrown at a target, would hit harder than the other because it has more KE. They won't, of course. While one has a lot more TOTAL KE than the other, much of that KE was not handed over to momentum to work with. So, both have the same amount of momentum directing exactly the same amount of KE at the target.

And you're wrong about the fly wheel. It's KE definitely is focused in one direction and it also has momentum. It's momentum is rotational vs linear. It's either going clockwise or counterclockwise. But if the flywheel explodes while it's in motion, the pieces of it fly off on whatever vector their momentum was oriented toward when the wheel broke.

The pieces, in fact, become projectiles. Like arrows. The only difference between arrows and the pieces of the flywheel is that the pieces of the flywheel take off on random vectors while WE control the vector momentum takes off on when we aim and launch our arrows.

Another example. Take a given quantity of gunpowder, say a hundred pounds. Place it on the ground next to a cannon shell and Ignite it. The expanding gasses have a great deal of KE, but that KE is expended in all directions. The cannon shell will be moved a few yards because only a small portion of the total KE was coming in it's direction. Take that hundred pounds of gunpowder and put it in a cannon barrel. Ignite it. The expanding gasses can only go one way and they can send the cannon shell miles downrange. The cannon barrel is like momentum in that it focus the gasses' KE in a given direction.

KE is capacity. Momentum is direction.

That's because when we aimed our arrow, we directed it's momentum to go in that specific direction.

Thank you Art.

I don't buy into the KE as defined by archery stuff.

You mean to say you directed its KE to go in a specific direction, which gives the momentum. You don't "direct" momentum. You use energy to impart a direction to mass.

You said "momentum is direction". Sort of. A better way to state it is: momentum is the description of energy in motion.

Even the tuning fork analogy has momentum. The legs of the fork have mass. The energy imparted to the fork makes the legs move. Moving mass has momentum.

Momentum is mass in motion. Direction is our use of it. The cannon ball has no momentum unless it is given energy the more you give it the farther it will go in what ever direction. Forcing it to go a certain direction is simply harnessing the momentum that is already there and using it. The straighter your arrow comes out of the bow the more momentum is focused. A sloppy barrel will not shoot a ball as far as a tight one. Wasted energy, but the same amount. Direction is not momentum. The better momentum is directed the more efficient energy is used.

We can only measure the initial K.E. of an arrow when shot from a bow by using a chronograph to get speed. We can't figure how much K.E. in the form of vibration is in the arrow while in flight, which I beleive is what Sylvan is talking about. Once in flight, energy is constantly being lost to distance, drag and gravity. Vibration in the arrow is also being lost in flight but has no bearing on how hard the arrow hits the target. If speed is lost, so is the momentum of the arrow. If I drop a hammer on the floor from 3 feet, there will be less momentum than if I apply downward force to the handle with my hand on the way down, from 3 feet also.

We can measure energy losses though. If you're brave and/or confident in your shooting abilities, you can put your chrono out at set distances and measure the speed.

The arrow weight is fixed- it's the same coming out of the bow as it is at 80yds. You can measure the initial velocity of the arrow coming out of the bow. That will tell you your initial KE value.

Then you can measure the speed of the arrow at the target at different distances. Say I put my chrono in front of the target at 60yds (being the brave person I am). I measure that speed out at 60yds. The weight is the same so now we know the KE value at 60yds.

Subtract the value at 60yds from the initial value, and that will be how much energy is lost from drag, vibration, inefficiency of the arrow to bow combo, etc.....

But you're right- you can't single out each loss factor and determine how much energy is being lost for each parameter by itself. (this amount for drag, this amount for arrow flex, this amount for improper flight)

No, you impart energy to a mass to produce motion. Rotational, orbital, elliptical. Once in motion, that object will have KE and momentum.

I know I didn't say it correctly on a technical basis, I was trying to make the concept simple enough to be understood. But still, momentum is a vector factor. A vector is direction. When we aim an arrow to go in a certain direction (or to say it another way, on a specific vector), then the whole MO/KE package goes with it.

It is more correct to say that momentum is a form of inertia. Newton's First Law of Motion. aka the law of inertia: "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force." The higher the mass the more inertia, so the more force it takes to move it. Also, the more force it takes to stop it when it's in motion.

The part we're interested in, when talking penetration, is keeping our arrows from stopping once they're in motion. Right? Notice that KE is not mentioned.

What are you saying no to? I think we agree on your statement.

Neither is mass. [8D]

Alright, now I love physics, but man; you guys have too much time. I don't think either the KE or the Momentum crowd is going to win this. THere are jsut too many circles being run here, after all we are talking about related terms.


IT is fun to read anyway