# Can this scenario produce enough energy to be useful?

• B
If I have a given weight, say 1,000 lbs, suspended 18" in the air, I have stored energy. If my understanding is correct, I can release it and it will fall quickly. If the line supporting it was wrapped around some form of generator I could theoretically light a bulb or something for a second. If there was some way to slow the fall it would light a proportionately dimmer bulb for a longer time. (Yes?/No?). Slowing the fall would cost energy, unless I could capture it in some other way. Am I correct that a stronger generator might slow the fall? Or, perhaps the fall of 1,000 lbs could raise five 100 lb weights 18", thereby slowing the fall by about half, and store the other half for later use?
Is my thinking out of line, or useless, or theoretically correct? Even more importantly, if I have this much weight (stored energy) is there anything I USEFUL that I would be able to do with it? I don't know why, but these questions have been bothering me, and I have no knowledge of physics and no one to whom I can turn for answers. But it seems like this is the right place to get some relief! Thanks for any answers.

fresh_42
Mentor
If I have a given weight, say 1,000 lbs, suspended 18" in the air, I have stored energy. If my understanding is correct, I can release it and it will fall quickly. If the line supporting it was wrapped around some form of generator I could theoretically light a bulb or something for a second. If there was some way to slow the fall it would light a proportionately dimmer bulb for a longer time. (Yes?/No?). Slowing the fall would cost energy, unless I could capture it in some other way. Am I correct that a stronger generator might slow the fall? Or, perhaps the fall of 1,000 lbs could raise five 100 lb weights 18", thereby slowing the fall by about half, and store the other half for later use?
Is my thinking out of line, or useless, or theoretically correct? Even more importantly, if I have this much weight (stored energy) is there anything I USEFUL that I would be able to do with it? I don't know why, but these questions have been bothering me, and I have no knowledge of physics and no one to whom I can turn for answers. But it seems like this is the right place to get some relief! Thanks for any answers.
This is basically how hydroelectric power plants work: Aswan Dam, Hoover Dam, Three Gorges Dam, and several hundreds more.

anorlunda
Staff Emeritus
Sure.

First, think of classical wind up clocks. Lifting a weight in the air was one of the primary ways to wind the clock.

Then there is this video.

But if your idea of USEFUL is recharging your Tesla car to drive 250 miles, forget it.

Edit: Use numbers, 1500 foot-pounds 0.0006 kWh of electric power (if I did the math right), which at ten cents/kWh is worth 0.006 cents.

CWatters
Homework Helper
Gold Member
I recall there is a project that uses special railway wagons to store energy. When there is surplus wind energy they use the electricity to power the train engine that pulls the train up a hill. When there is a shortage they let the train roll back down using the motor in reverse to generate electricity which is fed to the grid.

Some energy is lost as heat both on the way up and back down but apparently the overall efficiency is good enough.

I'll try and find a URL.

sophiecentaur
Gold Member
2020 Award
"Sysiphus" What a great name!

phinds
If there was some way to slow the fall it would light a proportionately dimmer bulb for a longer time. (Yes?/No?). Slowing the fall would cost energy, unless I could capture it in some other way. Am I correct that a stronger generator might slow the fall? Or, perhaps the fall of 1,000 lbs could raise five 100 lb weights 18", thereby slowing the fall by about half, and store the other half for later use?
Note that slowing the fall is the only way of getting any useful energy out of it. If you don't slow the fall, all the energy will become kinetic energy of the weight, and it will be lost when it slams into the floor. Fortunately any kind of generator will slow down, the weight. It may not slow it down enough however...
Your weight will produce a very large force, and if you just hook it up to a generator, you would need a large generator to slow down the weight sufficiently.
That's a problem if the power you want to use isn't large. You probably need some kind of gearing to reduce the force of the weight and increase the speed of the generator.

sophiecentaur
Gold Member
2020 Award
Note that slowing the fall is the only way of getting any useful energy out of it.
Also, it is no use using a brake to slow the fall; that would be inefficient. You need to use the appropriate gearing or, in the case of the rail system, the appropriate slope. There is a possible way around this if you actually stop the falling mass completely, every now and then. The average Power supplied can be as little as you want as long as your system allows you to smooth out the supply variations.
This is analogous to a Switch Mode Power Supply or regulator.

If I have a given weight, say 1,000 lbs, suspended 18" in the air, I have stored energy. ... Even more importantly, if I have this much weight (stored energy) is there anything I USEFUL that I would be able to do with it?

Anything useful? no, not really, unless you want to crush something. Once.

Use numbers, 1500 foot-pounds 0.0006 kWh of electric power

This. We don't have to guess, we can calculate the energy stored in the weight. This is the power of physics over speculation.

This is basically how hydroelectric power plants work: Aswan Dam, Hoover Dam, Three Gorges Dam, and several hundreds more.

Yes, but look at the difference between a dam and the OP's 1000 pound weight. Hoover dam is rated at something like 2000 MW (now that is useful). But the water flow through there is 30,000 to 40,000 cubic feet per second (say 2.5 million pounds per second) and it is falling almost 600 feet. That is one million of the OP's devices every second.

So, somewhere between the 1000 pound weight and the Hoover dam, the potential energy goes from "not useful" to "very useful." Where is the dividing line? Call the engineers

anorlunda
Staff Emeritus
It is silly to debate "useful". Someone it a hut may consider it useful to light a single LED for an hour.

gmax137 and russ_watters
Ibix
2020 Award
It is silly to debate "useful". Someone it a hut may consider it useful to light a single LED for an hour.
Indeed. As already noted, many clocks work by the OP's method, using an escapement to control the rate of energy output.

gmax137
@anorlunda and @Ibix -- I quite agree, "useful" is in the eye of the beholder. Though it may not have come through in my post, my real point is that we can quantify the stored energy [and then use that insight to say something about it's utility for a given situation].

CWatters
Homework Helper
Gold Member
If I have a given weight, say 1,000 lbs, suspended 18" in the air, I have stored energy.

1000lbs = 453kg
18" = 0.46m
g = 9.81 m/s/s

So by my reckoning you have approx. 453*0.46*9.81 = 2048 Joules of stored energy. 1 Joule is 1W for 1 second.

If the line supporting it was wrapped around some form of generator I could theoretically light a bulb or something for a second.

Assuming you had a 100% efficient generator you could light a 50W Incandescent bulb for about 40 seconds (50W*40S=2000J).

If there was some way to slow the fall it would light a proportionately dimmer bulb for a longer time. (Yes?/No?).

Yes. For example..

500W for 4 seconds
50W for 40 seconds
5W for 400 seconds
0.5W for 4000 seconds

or a typical 5mm diameter (0.015W) Led for about 37 hours (I must remember not to leave the TV on standby).

Just for info a human turning a hand cranked generator can generate a sustained power of about 50W. So it would take someone about 40 seconds of hand cranking to winch the weight back up again. Depending on the light bulb it might be better just to connect a hand cranked generator directly to the light bulb and do away with the weight.

Your mileage will vary as I've not allowed anything for losses due to friction, electrical resistance etc. In practice it might be reasonable to expect half these figures.

sophiecentaur
Gold Member
2020 Award
It is silly to debate "useful". Someone it a hut may consider it useful to light a single LED for an hour.
Exactly. For a grandfather clock, that few kg is immensely useful. The Sysiphus project has some really good points for storage of energy. In its eventual form, it can produce constant power out. That's very 'useful' and they quote actual figures.

I love forums! You guys have been very informative. Since I was a kid, I've thought about how to store energy for later use. The Sysiphus Project did catch my eye, and I loved the Gravity light concept. It seems to be doing exactly what I was thinking, except I was thinking of using 1,000 pounds instead of 10 pounds of rocks. Logic tells me that using that same idea, I could get about 100 times the light. I think I know a way to get the 1/2 ton up without using MY energy, although I haven't actually proved it. Before doing so, I wanted to know if it would be worth doing. I could replicate the effort a whole bunch of times to increase the yield, although it takes time to get the weight back up. I thought perhaps I could charge several 12v (car) batteries and keep them topped off for use when the power goes out. I will probably never DO anything, but I enjoy thinking of things like this. Thanks to everyone.

sophiecentaur
Gold Member
2020 Award
I will probably never DO anything,
We are all like that, to some degree and the 'Maths' can be quite a hurdle to get over. Most people are prepared to do some rough calculation, at least, where money is concerned. Precentage Interest rates and repayment rates make some sort of sense to us, almost intuitively. If you dig out your calculator, it's not hard to do the sums that are involved with methods of `Energy storage'. The Joule (kJ or MJ) is a very easy quantity to get familiar with. It's worth while calculating how many J is used up by a light bulb or heater and seeing how that compares with the J stored by lifting a given Mass up, say 100m. Play the Numbers Game. Forget about Efficiency, initially. That will make it easier.

1000lbs = 453kg
18" = 0.46m
g = 9.81 m/s/s

So by my reckoning you have approx. 453*0.46*9.81 = 2048 Joules of stored energy. 1 Joule is 1W for 1 second.

Assuming you had a 100% efficient generator you could light a 50W Incandescent bulb for about 40 seconds (50W*40S=2000J).

Yes. For example..

500W for 4 seconds
50W for 40 seconds
5W for 400 seconds
0.5W for 4000 seconds

or a typical 5mm diameter (0.015W) Led for about 37 hours (I must remember not to leave the TV on standby).

Just for info a human turning a hand cranked generator can generate a sustained power of about 50W. So it would take someone about 40 seconds of hand cranking to winch the weight back up again. Depending on the light bulb it might be better just to connect a hand cranked generator directly to the light bulb and do away with the weight.

Your mileage will vary as I've not allowed anything for losses due to friction, electrical resistance etc. In practice it might be reasonable to expect half these figures.
All the information received on this forum has been helpful, albeit confusing to me. I'm still grappling with the basic principles. If I understand your comments, if I made a generator fashioned like a child's top that spins faster the more (harder) you push down on the worm gear, drop the weight for 3/4 of a minute, I could only light a 50W bulb (minus loss for inefficiencies). Slowing the descent with gearing would reduce the wattage but increase the time. Is that correct? I have looked at the gravity light, mentioned in an earlier post, which appears to use all the physics I've been asking about. They are using 1/150 weight to light 3 LED's for 20 minutes. I don't really need to light 450 LED's for 20 minutes, but wondered if I was able to constantly re-load the weight, would I be able to recharge a bank of batteries for use during power outages, or off-grid living. I see the available weight as energy and wonder how I might use it. I've considered other ways of generating power using the weight, including somehow applying the weight to a vertical pipe filled with water that has an output valve, pushing the water out at a high pressure to push a waterwheel. I know there has to be some use for this energy. God didn't invent gravity just to keep my lazy butt in a chair!!

anorlunda
Staff Emeritus
The gravity principle can be used to supply the energy for a whole city. It is called pumped hydro. But instead of lifting a bucket of water by one meter, you need to lift an entire lake weighing millions of tons by the height of a mountain.

The bucket and the lake use the same principle. What you need is to pay more attention to the numbers, not just the principle.

Nugatory
sophiecentaur
Gold Member
2020 Award
All the information received on this forum has been helpful, albeit confusing to me. I'm still grappling with the basic principles. If I understand your comments, if I made a generator fashioned like a child's top that spins faster the more (harder) you push down on the worm gear, drop the weight for 3/4 of a minute, I could only light a 50W bulb (minus loss for inefficiencies). Slowing the descent with gearing would reduce the wattage but increase the time. Is that correct? I have looked at the gravity light, mentioned in an earlier post, which appears to use all the physics I've been asking about. They are using 1/150 weight to light 3 LED's for 20 minutes. I don't really need to light 450 LED's for 20 minutes, but wondered if I was able to constantly re-load the weight, would I be able to recharge a bank of batteries for use during power outages, or off-grid living. I see the available weight as energy and wonder how I might use it. I've considered other ways of generating power using the weight, including somehow applying the weight to a vertical pipe filled with water that has an output valve, pushing the water out at a high pressure to push a waterwheel. I know there has to be some use for this energy. God didn't invent gravity just to keep my lazy butt in a chair!!
It's all to do with the numbers and they can often give non-intuitive answers. One of the big hurdles to get across is the significant difference between the Energy for doing Mechanical Work and the Energy used in Heating things up. Way back, they came up with a concept of The Mechanical Equivalent of Heat. This says 1 Calorie is the equivalent of 4.2 Joules of Work. Lifting 0.1kg one metre uses 1J (approx) and heating up 0.1kg of water by just 1°C would require 420J. Those two scenarios are easy to visualise and I think they show just how much more Work / Energy is involved for heating than 'doing things'. Use a hand drill to make a hole in a piece of metal. You can wear yourself out doing that but find that the metal has hardly warmed up at all.

CWatters
Homework Helper
Gold Member
All the information received on this forum has been helpful, albeit confusing to me. I'm still grappling with the basic principles. If I understand your comments, if I made a generator fashioned like a child's top that spins faster the more (harder) you push down on the worm gear, drop the weight for 3/4 of a minute, I could only light a 50W bulb (minus loss for inefficiencies). Slowing the descent with gearing would reduce the wattage but increase the time. Is that correct?

Gearing alone won't slow the descent. You need the right combination of load (eg lights connected to a generator and the right gearing). What will happen is that the weight will accelerate downwards causing the generator to spin faster and faster increasing the voltage to the lamps causing them to consume more and more power. The system will reach an equilibrium when the weight is losing potential energy at roughly the same rate as it's consumed by the lights.

I have looked at the gravity light, mentioned in an earlier post, which appears to use all the physics I've been asking about. They are using 1/150 weight to light 3 LED's for 20 minutes. I don't really need to light 450 LED's for 20 minutes, but wondered if I was able to constantly re-load the weight, would I be able to recharge a bank of batteries for use during power outages, or off-grid living.

Sure. Just calculate how much energy you need to store and you can work out how much weight and height you need. Others have already mentioned that this is exactly how pumped storage works.

I see the available weight as energy and wonder how I might use it. I've considered other ways of generating power using the weight, including somehow applying the weight to a vertical pipe filled with water that has an output valve, pushing the water out at a high pressure to push a waterwheel. I know there has to be some use for this energy. God didn't invent gravity just to keep my lazy butt in a chair!!

In the long term the ultimate source of the energy isn't gravity. It's whatever you use to "re-load" the weight. Hydroelectric power stations use the sun to evaporate water, raise it up forming clouds then rain refills the reservoir again. So the ultimate source is the sun.