Momentum Not Conserved, in the use of a medieval battle flail

What an idiot.

This post has been edited by ubavontuba on Sep 1 2007, 01:32 AM

No, I was somewhat dismayed by it actually, it demonstrates a fundamental misunderstanding of the idea of potential energy versus actualized energy.

Actualized energy, for example, kinetic energy, is what's involved when you do work (work being the product of force and distance).

Potential energy represents the potential to do work.

For example, take the pendulum.

At the bottom of the swing it has a ceartain amount of kinetic energy, we can calculate this by measuring it's instaneous velocity, and using ke=1/2mv^2.

As the pendulum moves through it's upstroke, it does work to overcome gravity.
The work done is 'stored' in the pendulum as gravitational potential energy.

At the top of it's swing, in that instant when it hangs there, with zero velocity, it has zero kinetic energy, but it's gravitational potential energy (governed by gpe=mgh) is at a maximum.

It doesn't matter if you then slide a table under it to stop it from falling, it still has the potential to fall, the potential to do work, and the potential to have kinetic energy. This is the essence of all forms of potential energy.

The first equation that Dallas posted is the essence of the conservation of energy "When you sum up all of the available forms of energy, potential, and actualized, they are a constant, and remain constant." In other words "Energy can neither be created nor destroyed, merely transformed."

Your thought experiemnt, let us take, for example, to electrons.

The electrons experience a repulsive force.
The force is inversely proportional to the distance between them IE the smaller the distance, the greater the force.
If you set one electron in motion towards the other at velocity v, in order to do so, you must apply a force, this force must be sufficient to overcome the repulsive force of the other electron at the distance they are at, as the distance decreases, the electron experiences an increasing repulsive force, which has the effect of decelerating the electron.
The electron in motion, therefor has a maximum in kinetic energy at the moment that you set it in motion.
After the electron has reached the point where it's velocity is zero, that is when the potential energy of the electron is at it's maximum. The potential energy is being caused by a repulsive electric field, so we call it electric potential energy.
At this point, the electron has the potential to do work, it has the potential for moving, and it has the potential for possesing kinetic energy.
This potential is more then it had before, and the reason it is more then it was before is because work was done in overcoming the electrostatic repulsion to get it to it's new position, and in so doing, energy was 'put into' it.
It doesn't matter if you block the path of the elctrons so they can't move apart from each other, the repulsive force exists between them, therefore they still have the potential to do work, have a velocity, and have kinetic energy.

This post has been edited by Trippy on Sep 1 2007, 01:48 AM

Yes, you sure are. And you back it up with every post, over_the_hill_Eric.

Bummer. And I thought I was being clever...

Newton called this "force."

Kinetic energy isn't always actualized though. Often, it's expressed as "potential kinetic energy" (as in the case where two approaching mass vectors do not intersect).

But is the potential really there, when there is no real potential for relative motion? For instance, does a small rock's surface molecules really have a potential to fall into the center of the rock? What if it's orbiting in Saturn's rings, where gravity can't hold anything together? What if it's in an eliptical orbit that brings it out far enough for gravity to start working again, and then back in?

Agreed.

As I stated, storage is the essence of potential energy.

Agreed.

What if you simply filled a hole in a featureless globe that the pendulum was swinging in? A place where all matter has fallen to it's lowest potential?

Right. As I flippantly stated it: "Energy equals energy."

That's not the same at all. The electrons can never collide, therefore they can store the potential and push each other away again. That doesn't happen in my thought experiment. the collision happens, and the potential energy storage ceases to exist. The system becomes one, single mass.

This post has been edited by ubavontuba on Sep 1 2007, 02:18 AM

The list of "Eric_the_uvavonbozo idiocies" keeps getting longer

Take a simple collision.

Two lumps of plasticine of mass m, moving towards each other with velocity v, over a surface they experience no friction with.

Total mass of the system is 2m, and the center of mass lies exactly halfway between them.

If we draw a set of cartesian axis, centered on the center of mass, then we can say that m1 has velocity -v and has an arbitrary point +x, we can also say that m2 has velocity +v and an arbitrary starting point - x.

Before the collision, the total momentum of the system:

ptot=(m1 + m2).(v1+v2)
Ptot=2m.(v-v)
Ptot=2m.0
Ptot=0

After the collision, the velocity of the mass is 0, so the momentum is 0.

Thus, momentum is conserved.

As far as kinetic energy goes, collisions generate compression, compression causes heat, heat is a form of energy. They also generate a small amount of sound, through the compression of air in the space between them, and by the propogation of shockwaves within the final resultant mass.

If you were to sum up all of the forms of energy 'released' by the collision, you would find that, as Dallas stated with his first formula, they are equal tosome number, and that number is the same as the sum of the kinetic energies of the original mass.

As far as the idea of dropping something on something much larger goes (for example, an asteroid on the earth) then exactly the same argument applies. The difference is that if the asteroid is 'stationary' at 'infinity' and allowed to accelerate towards the earth, then when it is stationary, it has a maximum potential energy, as it falls it accelerates, this acceleration converts it's GPE into kinetic energy. In this respect, the considerations are identical when dealing with a pendulum. The considerations required for the colision between the earth and the bolide are the same as the above example using the plasticine.

Edit: Also worth noting is that the earth also falls towards the asteroid, but that because the earth is many times larger then the asteroid, the acceleration of the earth towards the asteroid is many times smaller.

Part of where you seem to be going wrong, is that you seem to have a fundamental misunderstanding of what a force is.

Work is the product of force and distance, not force. Newton did not, as you caim call work a force.

Observe:

f=m.a
f=kg.m/s^2

w=f.d
w=(kg.m/s^2).m
w=kg.m^2/s^2

Notice that work and force have different units, therefore, they measure different things.

Now, also observe:
ke=1/2 mv^2
ke=kg.(m/s)^2
ke=kg.m^2/s^2

Notice how Kinetic energy and work have the same units? They're different ways of measuring the same thing, when you do work, you're putting energy into the system.

Note that:

gpe=mgh
gpe=kg.(m/s^2).m
gpe=kg.m^2/s^2

So work is a measure of energy, not force.

This post has been edited by Trippy on Sep 1 2007, 04:35 AM

Again, that's not the same. That's just a simple collision. There's no potential energy storage resulting from an included (in the system) conserved force. Also, I have no argument with the conservation of momentum in a basic collision scenario.

Right. Heat is a result of kinetic energy collisions. But by including an atmosphere, you open the system. It's no longer isolated.

Yes, but only at the point of collision. My contention is that a conserved force can change the KE in the system before the collision occurs.

You're right above, but it's still not a model that's consistent with my thought experiment.

Hmm. A misunderstanding. I was stating that actualized energy is equivalent to force, not work. For instance, you can apply force (using actualized energy) and perform no work.

As I stated above, a misunderstanding.

You stated:

• "Actualized energy, for example, kinetic energy, is what's involved when you do work (work being the product of force and distance)."

"Actualized energy" seems to me to clearly be the subject, so I reponded with:

• "Newton called this 'force.'"

This post has been edited by ubavontuba on Sep 1 2007, 08:18 PM

Untrue. The atmosphere is part of the earth-bolide system.
The atmosphere applies a braking force.
The atmosphere, through the braking force converts Kinetic energy, and Gravitational Potential Energy to Heat energy and Chemical Potential Energy.
If you were to measure the KE, GPE, Heat Energy, and CPE of the bolide, and the atmosphere as the bolide passes through the atmosphere, you would find that at all times they add up to a constant.

You need to get this term 'conserved force' out of your head, it's a rubbish term.

Oookay...

Are you saying that this isn't your thought experiemnt?

IN your thought experiment we have:
Two bodies - These would be the earth and the Bolide, so check.
An attractive force - This would be gravity, so check (gravity is even one of two you mention).
A non-elastic collision - check, the collision between a bolide and the earth is non-elastic.

So, my scenario contains every element of your thought experiment, and yet you're claiming that it doesn't represent your tought experiment?

Maybe you need to think about this some more, and then state in plain, unambiguous terms, precisely what the experimental setup is in your thought experiment.

Obviously you missed the point of my dimensional analysis.
Force is measured in kg.m.s⁻²
Energy is measured in kg.m².s⁻²
Therefore what I referred to as "Actualized energy" is not what Newton called force.
They're measure different things, therefore, they are different things.

This post has been edited by Trippy on Sep 1 2007, 09:04 PM

I don't want to get into a pissing contest, but unless we can resolve our communications problem, I don't see how we might proceed.

You stated:

"Actualized energy, for example, kinetic energy, is what's involved when you do work (work being the product of force and distance)."

It clearly states that "actualized energy" is what's involved when you do work. You then go on to state (in parenthesis) that work is the product of force and distance.

Therefore, logically, "actualized energy" can be one of two things... force, or distance. Which is it? It seems it clearly cannot be distance (unless you think a kilometer is a form of energy!). Therefore "actualized energy," is force.

Where is the flaw in my logic?

This post has been edited by ubavontuba on Sep 1 2007, 09:26 PM

it's neither.

As i clearly stated, Work is a form of energy.
Work is the product of force and distance.
You can't ask if it comes from the force, or comes from the distance, because it's neither.

Again, this was the point of the dimensional analysis.

Energy is neither force, nor distance, but what happens when you combine them.

If I push on a frictionless object, I apply a force to that object, and the object accelerates while I am pushing it, when I stop pushing it, the object stops accelerating and travels in a straight line at a constant speed until something acts upon it to change one of those two things.

At the same time, I am applying a force to the object as it travels a certain distance. In applying force over that distance I am doing work. The object having work done upon it gains kinetic energy.

It's the same when I lift an object. I am applying a force to that object to move it through a distance, the distance being the height that I lift it. The work that I do when I lift it, is the same as the difference between the GPE in it's old state and it's new state.

Perhaps a more accurate statement might have been that "Work done represents a change in energy, and is the product of force applied over a distance" but I didn't particularly feel like getting into the myriad of situations where work is done, but it's not obvious where the change of energy is.

No. You clearly stated, "Actualized energy... is what's involved when you do work." That's not the same as saying it is work.

Motion/work.

Not telling me anything new.

Only relative to some other point of reference.

Sure.

I simply liked "actualized energy" as meaning an applied force. It simply makes sense. Now, it's gibberish.

This post has been edited by ubavontuba on Sep 1 2007, 11:14 PM

My patience is rapidly wearing thin.

I'm beginning to think that Dallas and 5Donut were right about you.

What, precisely, do you think work is, if it's not the process of doing work?

Christ on a crutch, this is stuff they teach to 15 year olds in my part of the world.

I don't get what's so hard about this to understand.

I think:

Work equals work.

Force equals force.

Mass equals mass.

Energy equals energy.

Distance equals distance.

Actualized energy apparently equals work, dependent on how you feel at the moment though.

I think the phrase "actualized energy" should be dropped from the discussion.

I don't appreciate your slights. If you can't be mature about it, leave.

This post has been edited by ubavontuba on Sep 1 2007, 11:13 PM

Work is energy. I proved this with the dimensional analysis. You did understand what that meant didn't you?

I used the phrase "Actualized energy" to distinguish between...

Well, I suppose to distinguish between energy that is doing work or represents work done, and potential energy which is the potential for work. I have been consistent with my application of the term.

Work is a force applied over a distance.
Work is energy.
Energy is force applied over a distance.
Force is acceleration applied to a mass.

Again, this was the entire point of the dimensional analysis that I performed exclusively for your benefit.

Gravitational Potential Energy, Kinetic Energy, and Work are all measured in the same units.

These units are different from the units that force is measured in.

I don't appreciate your obstinance, and your, well, lying seems an appropriate word. The way you keep focusing on trivialities that with a little application of context should resolve themselves.

My statement that the nature of work and energy is somethign that gets taught to 15 yearolds over here isn't a baseless insult, it's a statement of fact, I could even point you in the direction of the approriate curriculum documents, but somehow, I imagine you're not actually interested.

This isn't your thread, even though you've managed to hijack it and drag it off topic.

I've demonstrated my level of understanding of physics.
That I understand what the equations mean, and can perform dimensional analysis.

I have explained to you in clear, plain english where you're assumptions and conclusions around your thought experiement are in error.

If you can't be inteligent and thoughtful about it, then I suggest that maybe it's you that should leave.

Don't be flip.

The correct definition of "work" (in mechanics) is something like: The amount of energy transferred through the application of force.

It is a scalar quantity.

I recommend dropping that phrase. You can substitute the word "work" when you mean work and "energy" when you mean energy and "potential energy" when you mean potential energy. I know the differences.

More accurately, work is a transference of energy through the application of force.

No, that is work. Energy is the potential to do work. You can measure the energy transference during work, but energy isn't necessarily defined by work.

That one's okay.

I appreciate your effort.

Okay.

It seems to me to be quite apparent that your definition of terms and mine are different. I'm using the standards. What are you using?

I'm always interested in educational materials. My son just received a perfect score on his academic assessment tests. That doesn't happen without parental involvement.

I never said it was. Maybe I should be the one to leave then.

I think you're pretty smart. If I didn't, I wouldn't spend my time with you (considering your apparent lack of civility).

I disagree. You've beat around the bushes, but you haven't hit the squirrel yet. That is to say, I'm not sure you understand my premise. Maybe we should work to define it before we get into anymore esoteric definitions.

That's all I'm asking of you.

This post has been edited by ubavontuba on Sep 2 2007, 07:41 PM