Atwoods Analysis

[Wubbly]

I don't need to see pequaide's equations because I probably wouldn't understand them anyway, or his logic in applying them. It took me eight months to understand a simple Atwoods. How can I ever expect to understand cylinder and spheres?

@greendoor. I did not say a pendulum 'disproves' conservation of momentum, I don't yet understand why it's relevant. You start out with a zero value and end up with a zero value. Seems kind of boring. The momentum at the bottom of the pendulum swing is not what's doing the work to get it back up to where it started. The energy concept is more interesting. You have potential for movement, then you have movement, then you have potential again. Much more interesting.

Why can't we use springs? Is it because a spring is a valid tool for capturing energy and if we use springs they might show there is no extra energy?

It is perfectly acceptable to use a spring.

The kinetic energy equation derives from the work = force x distance equation by plugging in equations for uniformly accelerated motion, so of course they are self referencing. So what? If I perform work on an object by accelerating it, it now has energy. Why does it have energy? Because I performed work on it. It's not a problem that they are self referencing. If you forgot where the kinetic energy equation comes from, refer to your "summing velocity" thread where it was explained to you.

There is no need to convert momentum into Energy and compare Apples with Oranges.

At least you are admitting that energy and momentum are two different things. If you are going to reset your system, and your energy increase is based on your momentum numbers, then yes, at some point you will have to convert your momentum back into energy. Maybe this is where the output of the cylinder and spheres comes into play.

[Wubbly]

I went into this experiment with an open mind. I was only about 70% certain what I would find. The spreadsheet was so clear in what it was saying, but I didn't know how the actual experiment would respond to the numbers.

In the first few experiments I performed, it showed a relatively constant spring length value, but the numbers varied by about an inch from the high to the low. I didn't know what my mass numbers were so I went and bought a small kitchen scale and found that I was randomly pairing masses together that weren't too similar. The 2.5 pound weights are not exactly 2.5 pounds. After weighing each mass and pairing the similar masses, the results looked much better. It also helped to replace some of the strings with wires, and putting a block of wood under the spring, so it didn't have to lift before it stretched, reduced oscillations. After these changes the results looked much better. I was actually shocked at how closely the shape of the spring length vs mass graph resembled the kinetic energy graph. That little blip down on experiment 5 was probably because for experiment 5 I didn't have 2 masses that were perfectly close in mass. I didn't feel like going to Wal-Mart and buying a few more weights in hopes that I would find two that were closer together, figuring the results were good enough and those who choose not to believe the results weren't going to believe them anyway.

I believe in the Atwoods equations. I believe that momentum is increasing while kinetic energy remains constant. What I don't believe is that the huge momentum increase translates into excess energy or has the ability to reset the system. The theoretical momentum increased by 149%, yet somehow this supposedly real and measurable quantity did not show up in the experiment. I did not believe momentum was an accounting tool until now. This experiment opened my eyes.

Attached is a spreadsheet showing the results of the Atwoods Stop Drop experiment. The spring showed a relatively constant value as predicted by the energy equation.

[Fletcher]

Greendoor. You certainly have a way with words. You summed up the alternate viewpoint quite clearly, (unless you think Fletcher has the 'alternate' viewpoint).

I know there is no law of conservation of kinetic energy, but there is a law of conservation of energy. Also, potential energy can be exchanged for kinetic energy, and kinetic energy can be exchanged for potential energy. A pendulum shows this.

If you look at a pendulum from a momentum perspective, at its highest point it has zero momentum, at its lowest point it has maximum momentum, and as it swings back up it looses momentum back down to zero. A pendulum is not an example of conservation of momentum because the value ranges from zero, to some maximum value, back to zero. (I am not talking about a Newtons cradle where both momentum and kinetic energy come into play).

If you look at a pendulum from an energy perspective, at its highest point it has maximum potential energy and zero kinetic energy, as it swings down it exchanges potential energy for kinetic energy. At its lowest point it has maximum kinetic energy and zero potential energy, and as it swings back up it reverses the swap, exchanging kinetic energy for potential energy. At its highest point it then again has zero kinetic energy and maximum potential energy.

If you had a perfect spring with no losses, at the lowest point of the pendumum you could capture the motion and store it. Would you be capturing the momentum or the kinetic energy? You would be capturing the kinetic energy and storing it as mechanical potential energy. You could then release the energy back into the pendulum and it would return to its original height. A spring is a force, distance relationship which conveniently results in Joules. Of course we don't live in a world where perfect springs exist and we have to deal with frictional losses, but this is no reason to disregard an energy approach.

Mass is a measurable quantity. Velocity is a measurable quantity. Is mass x velocity measurable or calculable only? Hmmmmm.
Mass is a measurable quantity. Velocity squared is probably not a measurable quantity. Is 1/2 mass x velocity squared measurable or calculable only? Hmmmmm.

I don't have WM2D so I can't test this with simulation software. My guess is that it would conserve energy. I am working on an experiment to stop an Atwoods machine using a spring. I'll use mass in increments of 2.5 lbs (1.13 kg) since that's the smallest weight I could find at Wal-Mart. I can probably do the first 5 or 6 iterations of the spreadsheet. After a preset drop, a spring will engage and bring the system to a stop. By measuring the spring stretch for each iteration I should be able to see either an increasing value, a constant value, or inconclusive results.

Wubbly, you are basically proving why a spring won't work - in that a spring stores energy, not momentum.

Momentum = Force x Time. Not Force x Displacement (which is what a spring does). A spring can lose the momentum you gain in your Atwoods, and deliver Energy.

Momentum and Energy can never be the same - they are both mathematical tricks with different formula. There can be some degree of crossover, but do not be fooled: Energy can be lost, very easily. It can also be created just as easily, as Pequaide repeatedly demonstrates. It's a numbers game.

A pendulum - mass and velocity at it's simplest.

I disagree that a pendulum disproves Conservation of Momentum. I think it proves it very nicely. I believe you are forgetting that Momentum is a Vector, unlike Energy which is a Scalar. In a pendulum you have two opposing Impulses - the Momentum of the bob which was gained during free-fall, and the force of gravity acting on the mass which is opposing this momentum as the bob rises. At any point in the pendulum swing, the sum of the two opposing Impulses (Force x Time) very neatly prove that Momentum is always conserved.

There is no need to convert momentum into Energy and compare Apples with Oranges. You can compare Apples with Apples and make just as much sense of what is actually happening.

A pendulum does IMO show the conservation of momentum - it has something called 'sway' - that is where the pendulum exerts a force on the pivot & the pivot is stressed or moves ever so slightly to compensate - a way to prove this is by having the pivot on a horizontal trolley - the pivot will move towards the pendulum & oscillate - the pendulum is quickly dampened/deadened & stops - if you want to increase the amplitude of the swing you apply an opposing force to the trolley & that energy input is directly translated into increased Pe of height.

The point is that using a fixed pivot you have momentum at any point - this can be used to do work - the amount of work it can do is dependent on the 1/2 mass & velocity squared relationship, not the mass times velocity relationship that momentum gives - this is correct because two different masses can have the same momentum but different ability's to do a measured amount of work [move an object] - otherwise two scenario's with the same momentum but different masses would do the same amount of work & they do not.

[Wubbly]

So where are the experimental proofs?

Here they are: http://www.youtube.com/watch?v=mFckbrrFvVs

None so blind as those who refuse to see.

Indeed.

[Fletcher]

Enjoyed your other U-tube lecture wubbly [Lec 29].

[Wubbly]

Enjoyed your other U-tube lecture wubbly [Lec 29].

LOL :)

[nicbordeaux]

hmmmm... rather instructive. Tell me wubbly, are the brains here so great (a few obvious ones realy are brains) that you must rise to defend your knowledge ? Or did you commit some unspeakable act in a previous incarnation ?

[Fletcher]

FYI wubbly - WM2D seems to follow your analysis in your Lecture 29 re. 1D two masses etc - I guess it's based on math logarithms as I would expect it to be ?

That is, conservation of energy rules if perfectly inelastic & conservation of momentum if perfectly elastic collisions - that would suggest that since you can set the elasticity values in the program [0 to 1.0 & any range in between] then the program outputs according to a combination of both depending on the elasticity values & the mass differences, since nothing is ever perfectly elastic or inelastic either way.

Atwoods seems to mimic a perfectly inelastic collision where you get Ke conservation [no surprises there] - as soon as you try to use that energy of the moving masses to lever, springboard, or whatever another mass you loose efficiency because of the elasticity qualities of the materials used - as you say in your lecture, for a perfectly inelastic scenario [perfect spring] you get the maximum value to Ke - anything less & you loose mechanical potential & the Ke changes, the difference being heat.

[Wubbly]

Nic, some people on this forum are very intelligent, however just because someone is intelligent doesn't mean they can't 'believe' odd things. I was investigating the 'belief' that huge amounts of excess momentum can somehow equate into excess energy in a system. I was playing with an Atwoods spreadsheet when I noticed something quite odd. The momentum was increasing while the KE flatlined. I thought it would be an easy enough experiment to do, but it turned out to be quite difficult. I have no idea what I may or may not have done in a previous incarnation.

Fletcher, It appears that if you tag a video with the correct keywords, youtube will display related videos along the right side of the screen. I must have picked the correct keywords because the MIT lecture relating to the information also displayed. That wasn't planned. If I had picked keywords of "dead cat" something else would be showing up. (I think John Collins needs to tag his video with the keywords "Bessler Wheel" because if I search on "Bessler Wheel" in youtube, his video doesn't show up.)

[pequaide]

Wubbly; I believe that you have forgotten that distance traveled is a function of time. The heavier Atwood’s are moving slower than the light Atwood’s. When the spring begins applying force to the moving heavy Atwood’s it has a longer time to apply the same force. I think the d = 1/2at² formula would be the appropriate one to use. The acceleration (deceleration) of the heavy Atwood’s would be smaller but that would be compensated for by the fact that the force works for a longer period of time.
I think you need to add time as a factor in your experiment. The same force applied to 3 times the momentum will have a lower acceleration but that will be compensated for by an increase in time.
Compress a spring. Tie it in the compressed state with a string that can be melted. Suspend the spring as in a pendulum. Place a large pendulum bob touching one end of the spring and a small pendulum bob touching the other end of the spring. Melt the string that is holding the spring in the compresses state. The opening spring will accelerate both bobs and each will achieve the same quantity of momentum. This is because the force in the spring is equal in both direction, ( Newton ’s Third Law of Motion) and the time over which the force acts is equal (this brings the Second Law into play), F = ma.

Fix one end of the same spring and compress it with the swinging large bob, lock the spring in the compressed position. Place the small bob in front of the compressed spring and release it. You will not get conservation of momentum you will get conservation of energy. Why the difference between the two experiments?

Because the time over which the spring unloads on the small bob is shorter than the time it would take to unload on the large bob. Or the time it took to load the spring is longer than the time it takes to unload it. And the loading and unloading force is the same. Even though the fix experiment does not conserve momentum it does complies with Newton ’s Three Laws of Motion. This is the same principal as your Atwood’s experiment, time is a factor.

The time over which the force acts in the cylinder and spheres is the same for both the cylinder and the spheres so this experiment is like the first experiment (suspended spring experiment) not the second (fixed spring).

[John Collins]

(I think John Collins needs to tag his video with the keywords "Bessler Wheel" because if I search on "Bessler Wheel" in youtube, his video doesn't show up.)

You're right Wubbly and yet I have tagged it. For some reason my tags don't seem to work. I must be doing something wrong. Any suggestions very welcome.

JC

[greendoor]

Nic, some people on this forum are very intelligent, however just because someone is intelligent doesn't mean they can't 'believe' odd things. I was investigating the 'belief' that huge amounts of excess momentum can somehow equate into excess energy in a system. I was playing with an Atwoods spreadsheet when I noticed something quite odd. The momentum was increasing while the KE flatlined. I thought it would be an easy enough experiment to do, but it turned out to be quite difficult. I have no idea what I may or may not have done in a previous incarnation.

Wubbly - I should just ignore your rude comments, but what the heck.

Believing that Bessler wasn't a fraud is considered an 'odd thing' by most people, so we are all whack-jobs anyway.

I find your definition of 'odd' to be odd - because you say that you found your Atwoods spreadsheet to be 'quite odd'. WTF? The numbers are doing exactly what you asked them to do.

Kinetic Energy can be converted into Potential Energy which is measured as Height. Since you aren't changing Height in your spreadsheet, the Energy calculations will remain constant.

Most people here feel compelled to always convert everything into Energy - as though Energy or Height is the only thing that has value.

This effectively means that you have devalued Momentum in your mind. This mindset is so strong, that you can look at a spreadsheet that clearly shows multiplication of Momentum, and you assume that large amounts of Momentum are worthless and can achieve nothing of interest. If that's what you believe, that's what you will get.

Try to understand what Pequaide is saying about springs. It's hugely significant.

[nicbordeaux]

It appears in the end of video as if the test bench has been irreversibly dissasembled into small component parts hich will be spread, were one to pursue this logic, over a wide geographic area, so that further tests will be impossible ?

If not, and I was wondering about his, would a bigger initial freefall have any relevance ? And what would happen were one to keep the diff ratio between the weights constant ?

Whatever, congrats to Wubbly, it was an admirably well constructed apparatus. Just wish more people would build to test theory...

And yes Wubbly, you must have done something awful in a previous incarnation and now you suffer :)

[pequaide]

The acceleration of a spring is not uniform like gravity but I believe this principal is the same: The higher the acceleration the less time it takes to cover a certain distance.

You might do the experiment backwards and concentrate on finding the time. Stretch the spring out .460 meters to a 12 kg balanced (no added overbalance) Atwood’s at rest, and then to a 4 kg Atwood’s at rest and observe the time difference for the spring to return to its non-stretched position.

[neptune]

~Wubbly. Why would you need to go to wallmart to buy weghts. You obviously have some scales , Custom -make weights using plastic bags/coke cans/bottles/jars filled with sand/gravel/ soil/ nuts and bolts/water to the exact mass you require. I am no math genius , but an expert on not wasting money.