# Homework Help: Amplification of energy by a pulley and a a balance?

1. Nov 18, 2005

### eosphorus

i rise 10 tons 10 m up by lowering ten tons 10 m down with a pulley

i connect the weights with a weightless balance i use the oscillation of the balance to produce electricity with an electric motor, when the weights reach the initial altitude the process is repeated

can anyone explain whats wrong with this assumption?

2. Nov 18, 2005

### brewnog

When you use an electric motor to produce electricity, you are taking energy out of the system.

Besides, you'll always lose energy to friction, so you can't even break even.

3. Nov 19, 2005

### eosphorus

you dont know how a balance works, the key of it is the vertical fixed arm

consider a big balance with 10 tons in each side with 1 m vertical arm, it starts from total verticallity and try to stop from reaching horizontallity does that produce energy? and you put the weights workless by a pulley into a potential diference that can be as big as you want, one up the other down, but that the balance the bigger the potential energy difference the bigger the energy produced when levelling

some may say that if you have a 1 m vertical arm both weights lose 1 m of potential energy when they level

to avoid this take the weights 100 m up and 100 m down take 1 m vertical arm and to not lower the weights farther than their origin make one arm 99 meters and the other 101 m, this way the weigths will always have the same shared potential energy and the balance will still tend to horizontallity, not a perfect horizontallity but certainly wont be vertical

4. Nov 19, 2005

### brewnog

I know full well how a balance works, thank you!

How are you going to extract energy from this system? What makes the weights keep cycling up and down? How does your idea not violate the laws of thermodynamics?

5. Nov 19, 2005

There is nothing wrong with your assumption has long as you remain with your machine and continue the process above that you call "I rise". In this example, "you" are the external energy source, and what you are trying to do is determine if "amplification" of the energy you put into the system occurs where the oscillations at the focal point are used to produce energy. However, it is my understanding that in power amplifier of this type the process must involve two stages, e.g., two coupled systems. So, where are the "two stages" in your system that would allow the amplification to occur ?

6. Nov 19, 2005

### vaishakh

The energy is coming from the gravitational force.

Yes, the gravitational force can be explored as much as you need. You may have hard about hydroelectricity generation. The gravitational energy is used to produced force to rotate turbine there. Anyway 10 tons!!!!! And they in oscillatory motion!!!!! Just think on the tension you need on the string. Thus the string needs to be strong as well as flexible

7. Nov 19, 2005

### Danger

Vaishakh, how on Earth do you consider a hydroelectric generator a closed system? It doesn't pump its own water back up.

8. Nov 20, 2005

### eosphorus

this is an ideal thought experiment

i start at 0 altitude and have 10 tons 1000m up and ten tons 1000 m down

1 m above 0 i put an axe so the balance has 1 m vertical arm and 999m on the upper arm and 1001 m in the lower arm

the balance will level the weight to some extension it will produce huge ammounts of energy when passing from 2000m diference to almost 0, i store that energy

now i have the weights more levelled but i put them again 1000 m up and down with a pulley using almost no energy though now i have plenty and repeat the process of the balance closing the system

you may ask how i switch from weightless frictionless pulley to weightless balance but thats what ideal experiments are good for you can do it

9. Nov 20, 2005

### brewnog

No it won't. The centre of mass of the system lies below your datum, therefore the arms will not move, assuming the weights and the pivot are colinear.

Ahh, but you don't. Because the arms aren't equal length, the rising weight would not have travelled the same distance as the falling weight.

In order to get back to the situation they started off in, ie the same altitudes, you'll need the same amount of energy you generated while the weight was falling. (In fact, you'll need a little bit more to overcome your losses, but you know this).

Last edited: Nov 20, 2005
10. Nov 20, 2005

### brewnog

Yes, but you can't just magically put the water back into the top reservoir and start the cycle again!

11. Nov 20, 2005

### eosphorus

"No it won't. The centre of mass of the system lies below your datum, therefore the arms will not move, assuming the weights and the pivot are colinear"

the balance its not colinear it has one meter vertical arm

"Ahh, but you don't. Because the arms aren't equal length, the rising weight would not have travelled the same distance as the falling weight"

remember that the balance axe is 1 m above 0 level so when the balance oscillates and levels for a moment both weights will be at 0 level

"In order to get back to the situation they started off in, ie the same altitudes, you'll need the same amount of energy you generated while the weight was falling"

in the case of a 0 friction pulley i need 0 work to get to the initial position but a 1 m vertical arm huge balance produces huge ammounts of energy remember that both potential energies add 0 but for a balnce its not 0

i know all that about conservation of energy i just ask whats wrong with my assumption

12. Nov 20, 2005

### Staff: Mentor

As brewnog explained, the only way the balance could produce "huge amounts of energy" is if the gravitational PE of the masses were reduced by having them lower and raise different amounts. (Are you treating the balance as a simple lever?)

Not sure that you do. In any case, as usual, your description is so unclear that it's hard to pinpoint the incorrect assumption. Can you draw a picture of your device?

13. Nov 20, 2005

### vaishakh

yes of course oscilation is a tough part. i agree with all. i just said that gravitational energy is something that is a explored to generate electricity as also a standard one. infact to maintain os cilation in the wy euphorus explained is also a very tough part. i don't think this works in an ideal situation that the process goes once a jerk is given to the object in top position

14. Nov 20, 2005

### pervect

Staff Emeritus
OK, why don't you tell us what you've learned about energy? Basic defintions, measurement, it's relationship with work, that sort of thing.

15. Nov 21, 2005

### eosphorus

ill be more concise:

with an ideal pulley can i rise 10 tons 10 km up by lowering ten tons ten km down with neglectable energy?

now take a squared paper and draw in it a centered spot, thats the 0 altitude, 10 squares above put one weight and ten squares underneath it another and unite them by a line, the axe of the balance is 1 square above the 0 point and 1 square to the right, now unite the axe and the balance arm with an one square horizontal line

the balance has a vertical arm of 1 square or 1 km one top or left arm of 9 squares and one bottom or right arm of 11 squares

would produce this giantic balance energy when passing from verticallity to horizontallity?

would both weights ever be lower than the 0 point?

wouldnt both weights both reach 0 point in the balance oscillation?

would this energy produced by the balance be bigger than the energy used to rise and lower the weights with the pulley?no, whynot?

im not here to believe im here to learn, but no by faith but by questioning

16. Nov 21, 2005

### eosphorus

by the way i dissagree in the way you consider a balance does not produce energy when unlevelled because the potential energy adds 0

if you take a balance with a big vertical arm and rise a weight the same you lower the other weight it will produce energy when levelling, if not put your finger in the gearing

17. Nov 21, 2005

### ZapperZ

Staff Emeritus
I noticed that you avoided addresing pervect's question to you. Why is that? You claim to want to learn, but how are you supposed we could help you to learn when you refuse to reveal what you understand? You use the same "words" from physics, but from what I've seen in your postings so far (in this and other threads), you have a very different way of understanding what they mean.

Clearify what you understand. If not, you will ALWAYS misunderstand what others are trying to explain to you. Or is this problem not obvious?

Zz.

18. Nov 21, 2005

### Staff: Mentor

What's needed is clarity, not concision.

OK.

Much better. Now I understand what you're talking about.

No. First thing to realize is that when it swings from vertical to horizontal the net change in potential energy is zero. The initial vertical position has the center of mass at zero altitude; so does the final horizontal position. It will reach the horizontal position with zero kinetic energy. (The horizontal position is not the equilibrium position for this thing.)

No.

No. The midpoint of the oscillation will be where the center of mass (midway between the masses) is directly under the support point. This means that the midpoint is where the thing is at a 45 degree angle to the vertical, not when it's horizontal. Of course, the masses will have kinetic energy at this midpoint, since they have lost potential energy.

The only energy "produced" by the balance is that due to the masses falling to a lower position. Both masses fall the same distance. To reload the balance (if you use up the energy) you'd have to raise both masses--which takes energy to do. (Your pulley system won't help you, since both masses must be raised to their initial positions.)

Study this setup carefully and maybe you'll learn something about conservation of energy.

19. Nov 21, 2005

### pallidin

By no means am I trying to be rude here, but I think that you are missing the point(s) of the advise given. That's OK! As long as you are willing to learn, all is good.
What you are proposing involves a classic set of errors. Look at it this way: You set-up a weighted mechanical system and "prime" it to effect motion.
Fine. Easily done, and thus "one-shot" arrangements can be designed in an infinite number of ways. But, to internally "cycle" the effect is what you are after. So, you set-up an arrangement that attempts the cycling in such a way that it brings the system back into an "original state"

Well, in a "closed system" this can not happen. The closest one gets is a protracted "dampening", whereby each successive cycle is LESS in effect than the one before it and, most importantly, the cummulative effect is less than the original "priming" energy. So, in the end, I LOSE energy. If during the cycle I attempt to "tap" the effect to generate power, the dampening occurrs much more quickly, and I will still never extract more energy than was put into it.

Another common misconception is with regards to pulleys, levers and similar devices. I can easily construct a pulley arrangement that permits me to lift 1,000 lbs by applying only 1 lb of force. Seems like I'm getting more "out" of the system than I put in, doesn't it?
In reality, to lift that 1,000 lb weight 1-foot, I would have to pull the other end of that simple pulley system(at 1 lb of force) 1,000 feet !!!!!
With compound pulleys I can dramatically reduce the length required(the 1,000 feet) but it REQUIRES that I pull with much greater force.
So, with a compound pulley system, to lift 1,000 lbs 1-foot might only require 20-feet of draw, but demands 50-lbs of force apllied during that 20-foot draw.
If one does the math, one sees that NOTHING is gained. In true reality there are frcitional aspects to consider, such that one can never "break even" much less "gain"

20. Nov 21, 2005

### pervect

Staff Emeritus
pallidin - a nice reply, but I have the impression that eosphorus needs more work on the basics, like work = force * distance, and that work represents the change in energy of a system.

21. Nov 22, 2005

### Staff: Mentor

Your comments are of course true, but note that this is one error that eosphorus does not make. His pulley arrangement has one mass rise while the other lowers; ignoring friction loses, one can certainly raise and lower these balanced masses with no added energy. His error is more subtle--a misunderstanding of what actually happens when his "balance" is released from a vertical position.

22. Nov 22, 2005

### eosphorus

i would really appreciate you point me to my mistake because since ive drawn it in the paper a 1000 times and since i see it working i think ill build it

i have a squared paper with a 0 altitude reference, 10 squares above and 10 below i put two weights and the axe of the balnce 1 square up and right of the 0 point

i let it oscillate with no friction the balance will convert potential energy to kinetic and to potential, the balance will go from left to right and to left, from verticality to almost verticality to verticality and so on

the balance starts vertical and goes to almost vertical on the other side,now i connect the electric engine and as the balance tends to horizontallity i take the energy till the balance gets horizontal with both weights at 0 point altitude

whats exactly wrong with this last assumption?

23. Nov 22, 2005

### Staff: Mentor

You assume that the balance goes from vertical on one side to (almost) vertical on the other side. Not so! Remember, you made this balance lopsided by placing the pivot away from its center.

No matter how the balance swings, its center of mass will not rise higher than its initial position. The initial height of the CM is at the 0 position. As the balance swings down, the CM will reach its lowest position when it is directly beneath the pivot point: at this point the CM has dropped $\sqrt{2}$ squares from its initial position and the balance has its maximum kinetic energy. (The balance is at a 45 degree angle at this point.) The balance will continue to swinging until its CM is once again at a height of 0: this occurs when the balance is horizontal. Note that the balance will only swing until it's horizontal; it will it will never reach vertical on the other side: not enough energy! If there is no friction, the balance will oscillate between vertical and horizontal.

24. Nov 22, 2005

### eosphorus

my assumption is wrong as doc al pointed the balance will go from verticallity to horizontallity and to verticality and so on not to almost verticallity as i had assumed

25. Nov 23, 2005

### pallidin

Regardless of the dynamics of mechanical movement, it is important to understand that energy is required to "set-up" the event.
The question you must ask yourself is this:
Is the energy extracted greater than the the energy applied to "set-up" the event?
In ALL mechanical arrangments the answer is no.