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Best way to capture the energy of a falling object

  1. Mar 6, 2012 #1
    What's the best way to capture the energy produced by a falling object, and then use that energy to produce electricity for example ?

    Like for instance, I've been thinking of a kind of water mill where it would be an object falling on the blades instead of water, therefore making the mill turn, but it probably would not make it turn fast enough to produce electricity. So is there a more efficient way to capture the energy of a falling object ?

  2. jcsd
  3. Mar 6, 2012 #2


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    After the object (or objects) has fallen once, how do you extract any more energy?

    The reason watermills work is because they are taking advantage of the Earth doing work to continually lift the water back up (by evaporation). A watermill does not make energy, it steals it from Earth's natural processes.

    I don't know what object(s) you are thinking of that would fall with regularity.
  4. Mar 6, 2012 #3
    Well imagine a ball that falls from a few meters high and lands on a blade from the wheel. With its weight, it would make the wheel turn, right ? Now it probably would not make it turn fast enough to produce electricity, even with a gear-transmission box. But I used the water mill (or any other kind of wheel) as an example of a way to use the energy produced by the falling object (or ball). I'm wondering if there is any other way that would be more efficient ?
  5. Mar 6, 2012 #4


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    I get it but where do the balls come from?
    1] You have a limited number of them.
    2] Once they have fallen, they are of no use to your device anymore. You would have to do work to raise them up above the wheel again. It would use more energy to do so than you got out of the device.
  6. Mar 6, 2012 #5
    Oh sure, I know I wouldn't be able to produce extra energy, I'm just wondering if there is a way to capture the energy produced, independently of how the object would go back up. It's just a hypothetical question
  7. Mar 6, 2012 #6


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    Well, yes. Anything that catches the object to spin an axle would be able to produce a small amount of electricity - provided it could overcome the resistive force of the generator.
  8. Mar 6, 2012 #7
    Ok great, thanks!!
  9. Mar 6, 2012 #8


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    I feel the need to warn you that this sounds like a school assignment, and they are almost certainly looking for a solution more carefully thought out than this.
  10. Mar 6, 2012 #9
    Haha no it's not a school assignment, I'm a graduate student but not in physics or natural sciences at all, I'm just interested in physics and curious! Thanks for the replies
  11. Mar 12, 2012 #10
    You have the idea in your mind that the "windmill" wouldn't be able to extract energy from the ball. You're probably either thinking of a ball that's too light (ping-pong ball?), or a windmill that's sized too large (full-size waterwheel?). Or both.

    Just imagine a MUCH heavier ball (a cannonball) and a MUCH lighter windmill, and everything should fall into place :)
  12. Mar 13, 2012 #11
    The heart of your question seems to be "what is the most efficient method to convert kinetic energy to electrical energy?" I would say turbines (essentially watermills) connected to generators are the most efficient. If there were something else more efficient, engineers would use this something else in power plants instead of turbine-generator stack which is now used.
  13. Mar 13, 2012 #12


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    If you have a cylindrical magnet inside a coil and push it, it creates a voltage in the coil, so if you could use the falling ball to kick the magnet, that might be a fairly efficient way to convert the energy into electricity. Not sure of the details, tho.
  14. Oct 5, 2013 #13
    Possible solution


    I was just thinking about this concept recently. I'm more of a Computer Science person myself, but also interested in topics such as these.

    The US produces roughly 250 million tons of trash per year that go into landfills. http://science.howstuffworks.com/environmental/green-science/landfill.htm

    Apparently they are trucking and bulldozing this garbage into a big hole. This seems like an endless "stream" (river? dam?) of motion...

    What if you dropped the garbage into the hole (I don't know, through some sort of tube so that it is directed), and on the way down (exactly like a waterfall) its downward motion is captured by the generator / turbine / etc to produce electricity?

    The garbage still gets to where it needs to be, down the hole. There is no need to expend work to bring that garbage back up the hill...there is always more that is coming...at least for the foreseeable future!

    I look forward to your reply.

    It seems like it's about capturing motion that already exists in our world--the highways full of cars, etc, etc.

  15. Oct 5, 2013 #14


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    If we could capture all the energy released by dropping 250 million tons of trash into a hole ten meters deep, we'd get 2.5X1013 joules out. At current electricity rates, that's about about $100,000 dollars a year - no way that would justify the cost of installing generating equipment at every one of thousands of dumps and landfills across the country.
  16. Oct 5, 2013 #15


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    Burning the trash would release more energy. This is done already. But it discourages recycling, because once those garbage burning power plants are build, they require a steady flow of garbage to operate economically.
  17. Oct 7, 2013 #16


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    People are getting a bit too combative. I thought the OP question was perfectly fine.

    The answer depends on the specifics of the system. If you are talking about a single solid object, you can do a lot better than a turbine. You can just attach the object to a rope and run the rope over a pulley. Use the rope to do whatever work you want done. If you don't want to attach the object directly to a rope, you can attach the rope to a pan that catches the object. If you have a stream of objects, you can set up your pans in a wheel with some kind of catch that releases the weights at the bottom of the wheel.

    A turbine has blades that are designed to deflect a fluid around the blade, and is less efficient for a solid object (although it will work).
  18. Oct 7, 2013 #17


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    If you are dropping objects from a height onto the generating device (paddle wheel or whatever), two important challenges are in not dissipating energy in a high speed collision with the apparatus and in not leaving the objects with much residual kinetic energy.

    Khashishi's suggestion addresses these by having the objects drop slowly while carried by the apparatus. So there is no high speed collision to dissipate energy. This is [roughly] the mode of operation of a classical water wheel.

    One alternate solution would be dropping steel ball bearings onto a relatively massive paddle wheel that is moving at almost exactly half of the ball bearing impact speed. With an elastic collision, the rebound velocity of the ball bearing would be almost exactly zero. Almost all of its kinetic energy would have been deposited in the paddle wheel.
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