I Like this Idea: Electricity from gravity. DUH why didn't I think of this !

Wmd_2008, yeah, not every one has a car like that though. I think a better innovation would be a simple fuel cell that uses gasoline and is 75% efficient along with capacitors used as batteries.

It's not my intention to refute physics because we KNOW that energy cannot be produced by setting (and leaving) a weight on the ground.

BUT...can someone please enlighten me.

Obviously, compressing (or stretching) a spring requires energy. (If I do this with my hands I would have to press together or stretch out the spring, applying force all the time. If I stop, the spring will snap back in it's initial form. Agree? --- SIMPLE EXAMPLE).

If I put the spring on the ground now I can put a large weight on it and the spring is getting compressed by the FORCE OF GRAVITY which is applied to the mass. CORRECT?

In the same way as I will always have to supply physical energy to compress the spring with my hands, the weight is compressing the spring.

WHY can this energy not be harvested?

The argument there is no energy cannot fly since energy must be constantly being supplied to compress the spring. (Obviously, gravity will "pull down" the weight and there is no reason it would stop doing this)

The energy can also not all-of-a-sudden be "zero" since then the spring would not be compressed?

Anyone enlighten me....

@NoRulesAllowed

It would require that potential energy of the spring to bring it back to its original length. PE= .5kx^2, If you tap into that potential somewhere along the compression to generate electricity then the spring wont be able to go back to its relaxed state. I think that's what would happen, probably wrong. Bedlam knows more.

en.wikipedia.org...:HookesLawForSpring-English.png

NoRulesAllowed
It's not my intention to refute physics because we KNOW that energy cannot be produced by setting (and leaving) a weight on the ground.

BUT...can someone please enlighten me.

Obviously, compressing (or stretching) a spring requires energy. (If I do this with my hands I would have to press together or stretch out the spring, applying force all the time. If I stop, the spring will snap back in it's initial form. Agree? --- SIMPLE EXAMPLE).

If I put the spring on the ground now I can put a large weight on it and the spring is getting compressed by the FORCE OF GRAVITY which is applied to the mass. CORRECT?

In the same way as I will always have to supply physical energy to compress the spring with my hands, the weight is compressing the spring.

WHY can this energy not be harvested?

The argument there is no energy cannot fly since energy must be constantly being supplied to compress the spring. (Obviously, gravity will "pull down" the weight and there is no reason it would stop doing this)

The energy can also not all-of-a-sudden be "zero" since then the spring would not be compressed?

Anyone enlighten me....

Ok. You're confusing effort with energy. Yep, it might take a lot of effort to keep a spring compressed. But that's not energy flowing into the spring. Once you get that spring compressed, I could clip a retainer on it and then it will stay compressed. The retainer clip isn't supplying endless energy, it's a piece of metal.

If I have a compression spring, and place a mass on it, and the spring compresses but doesn't bottom out, you will find that the spring now has stored energy to the amount (drumroll) mgh, where h is the amount of compression. And that's all. The mass isn't supplying endless amounts of energy into the spring. You've traded some gravitational potential energy in lowering the mass due to the spring's compression. That is now stored in the spring. And that's all that's in the spring.

Springs store energy in their tension or compression. Once the compression is finished, that's all that goes in there. It doesn't require "energy" from the weight to keep it compressed. If you're doing it with your hand, it feels like you're having to use "energy" to keep the spring compressed. But you could get the same effect with a strap.

It occurs to me that the key to this concept actually netting more usable energy (than simply not implementing it) is stop signs.

Particularly stop signs on a down grade and possibly on level streets. Since everyone is required to stop there, why not try and recapture some of that kinetic energy that is currently (at least in traditional gas vehicles) 100% lost at stop signs. It would effectively act as a in-road regenerative braking system, helping to slow your car as you approach the stop sign while capturing that energy, as opposed to losing all of it as heat from braking or downshifting.

I think it's a great idea, but like all things of course must be applied appropriately, and still may or may not actually work in practice.

daskakik
I don't think so, the moment the tire drops 2cm, the engine would have to push a little harder to get out of that hole.

Bedlam
Not if you're using it on a flat.

I happen to write for a living and I do not starve, so I'm pretty sure the lack of comprehension displayed above is not due to insufficient clarity on my part. Besides, others in the thread seem to understand me pretty well.

I am not talking about using it on a flat. I am talking about turning a downhill gradient into a series of horizontal (or gently inclined) steps or terraces and installing a hydraulic pressure plate at the edge of each step, forming a kind of lift that carries the car wheels down to the next level. So (1) I am not talking about using this on level ground and (2) the lip, I repeat, has been eliminated.

Now, Bedlam: you're still touting this 'use the downhill momentum to push my car up the next hill' line. Yes, you can do that on a long, straight downgrade followed by an upgrade, so long as you don't have any slower traffic in front of you. But you can't do it in most urban or suburban situations, which is where traffic density is usually highest (and hence also the potential for this device). Reconsider your position based on real-world mechanics; nobody is trying to repeal the Second Law of Thermodynamics here.

You might as well have rolled onto a single plate that dropped the entire distance before you rolled off the other end, breaking it into little pieces just obfuscates the problem.

Actually, that's a great example: it proves my point. If you had a setup like that, I could drive up the hill with just enough residual momentum to get me over the crest and on to the plate — which in your scenario is now a hydraulic lift powered by the weight of my car, and which harvests energy from my descent. With my engine switched off or idling, I actually save gasoline.

Thanks for providing such a vivid conceptual proof of the soundness of the idea. I guess it takes a scientist... Unfortunately your scheme won't work in practice, because if you had a setup like that there would be a traffic jam at the top of the hill while cars queued to take their turn on the lift. Breaking the lift up into little pieces, as originally suggested, eliminates that problem.

As for this:

You insult me because I say it will vary and you think this is common knowledge. In the next, you insult me because you think it should NOT vary.

First: I think a careful reexamination of those two exchanges will show that my position is consistent in both cases. I really can't waste time re-explaining myself to somebody who clearly isn't reading with their full attention.

Second: I've heard of America's love affair with the automobile, but I hadn't realised it was so passionate that suggesting to an American that they may not drive a car would be taken as a personal insult.

And finally,

I assume you drive an automatic? I don't. I can just downshift, but I digress.

Downshifting kills momentum just as surely as braking does. But I'm sure you knew that already: a good driver knows that downshifting is a complement to braking, not a substitute for it.

By the way, you'd get even more free forward motion out of that downslope if you put your gears in neutral. Did you leave engine braking out of your calculations because you regard it as insignificant?

Astyanax
So (1) I am not talking about using this on level ground and (2) the lip, I repeat, has been eliminated.

I, however, was discussing both, alternately.

Now, Bedlam: you're still touting this 'use the downhill momemntum to push my car up the next hill' line.

If not, then as I also said, if you have a hybrid and brake you will charge your battery, and you can forget that with this. Also, if you just wanted to coast and not use gas on the downhill, that's out too.

Actually, that's a great example: it proves my point. If you had a setup like that, I could drive up the hill with just enough residual momentum to get me over the crest and on to the plate — which in your scenario is now a hydraulic lift powered by the weight of my car, and which harvests energy from my descent. With my engine switched off or idling, I actually save gasoline.

Thanks for providing such an conceptual proof that the idea is indeed sound. I guess it takes a scientist.

Actually, it doesn't. This energy collector means you'll be driving down the hill as well as up.

Unfortunately your scheme won't work in practice, because if you had a setup like that there would be a traffic jam at the top of the hill while cars queued to take their turn on the lift. Breaking the lift up into little pieces, as originally suggested, eliminates that problem.

It does, however, illustrate that you have no forward vector in a clearer manner. This is often the approach when solving physics problems. The math comes out the same. It's not a design attempt, it's a simplification of the variables.

Astyanax, this entire thing sucks. It poses as a collector of otherwise lost energy, but it's not. It's clumsy, inefficient, and unworkable. But it obviously sounds attractive to you. Don't expect to see it in use, though.

reply to post by Bedlam


My dear Bedlam: in mechanics class, cars do inded freewheel down one hill in order to gain momentum to help them up the next. I vaguely recall hearing this from my secondary school applied-maths master and thinking even then that the old boy might know his dynamics but didn't know much about cars.

In the real world, we keep our drivetrains engaged when in motion for reasons of safety and convenience. We do not end up saving much petrol on downslopes, because we keep our throttles slightly open most of the time even when going downhill. This is because engine braking retards the car in spite of the effect of gravity. We only lift off the throttle if the slope is very steep and we feel the car is going too fast for safety (or is exceeding the legal speed limit).

I have no idea whether an idea like this is genuinely practicable. It may be too inefficient or expensive to pay its way. Most probably it will fail because people don't like bumpy roads. But — in spite of your efforts — I am yet to be convinced that it will cause cars to burn more fuel if the system is built into descending carriageways.

Astyanax
reply to post by Bedlam

 

In the real world, we keep our drivetrains engaged when in motion for reasons of safety and convenience. We do not end up saving much petrol on downslopes, because we keep our throttles slightly open most of the time even when going downhill.

Do you have a fuel-use indicator on your car? The sort that gives you instantaneous MPG or KM per liter readouts? If so, start watching it going up and down slopes. An averaging one won't do.

Astyanax
I am not talking about using it on a flat.

Yes, but not all downhill grades are equal and, as pointed out earlier, if you want to keep and maybe even increase forward momentum to take an upcoming hill then even on a downhill grade the lip is still there.

Looking at it logically the lip is always there.

reply to post by daskakik

if you want to keep and maybe even increase forward momentum to take an upcoming hill...

On this, see my conversation with Bedlam immediately above. Your other objection is somewhat frivolous.

Astyanax
On this, see my conversation with Bedlam immediately above.

Saw it but I don't see what you are getting at. Cars with manual transmissions can be put in neutral when coasting downhill.

I also see that you point out that breaking doesn't happen on all downhill grades. That was my point. Drivers maintaining speed to take an upcoming hill are already using that energy. Slowing them down and forcing them to step on the accelerator to get up the next hill is making them burn more fuel.

Your other objection is somewhat frivolous.

Facts is facts.

reply to post by daskakik

Cars with manual transmissions can be put in neutral when coasting downhill.

And do you know anyone who actually does that?

Facts is facts

Relevant ones are, certainly.

Astyanax
And do you know anyone who actually does that?

Yes, lots.

Relevant ones are, certainly.

Irrelevent ones are as well which is why your choice of words was inaccurate.