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Ships, locks and potential energy

  1. Dec 8, 2009 #1
    Hello!

    I have been pondering about potential energy and ship locks.

    Big ships tend to have great mass but are also, hopefully, able to float.
    So I am wondering if the water in a lock gets compressed by the ship's force of gravity?

    If not, you could raise the ship by pumping water into the lock, hook the ship up to a crane,
    and then let the water out and you would have increased the potential energy of the ship.

    And since there's no such thing as free energy, there has to be a "catch" here...

    Any explanation would be appreciated!

    Thanks,
    Johannes O Ferstad
    Norway
     
  2. jcsd
  3. Dec 8, 2009 #2

    A.T.

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    ... yeah what could it be. The energy used by the pump maybe?
     
  4. Dec 8, 2009 #3

    russ_watters

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    There is no need for a pump. You just let water run into the lock from the higher surface.

    Since the ship is displacing (replacing) water, it makes no difference to the water whether the ship is there or not.
     
  5. Dec 8, 2009 #4

    sophiecentaur

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    In a lock, you don't so much 'pump' the water in as using the energy from the water cycle (ah, back to School!) to provide the upstream water's PE to lift the ship as it flows into the lock - transferring PE to it as it rises.
    The ship's potential is the same, btw, whether it's floating in the water or hanging on a crane, if it's at the same height above the centre of the Earth.
     
  6. Dec 8, 2009 #5

    A.T.

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    I know there is no pump in a normal lock, but the OP mentioned pumping water into the lock. In a normal lock you convert some potential energy of the water on the up side, into potential energy of the ship.
     
  7. Dec 8, 2009 #6
    Thanks for answering, but it didn't really answer my question.

    You could theoretically (if there was such a thing as "no energy-loss") get the energy you used to pump the water back (using a turbine for example), and the boat would still be hanging in the crane..

    sophiecentaur: do you mean that they use water from the "upper" level to fill the lock, thereby elevating the ship?
     
  8. Dec 8, 2009 #7

    sophiecentaur

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    Usually, the upper section of canal is kept full of water by natural streams which are arranged to top it up. We're talking Heyday of Victorian Engineering. If the lock is into a Marina, then the water level will be topped up at high tide. I don't think anyone would pump water when not absolutely necessary.
    In times of drought, they often 'close' the locks to conserve water and wait till they are full of boats before locking up or down. Each filling represents a lot of water - a significant percentage of the total volume for a short length of canal (e.g. where the canal ascends a hill in short stages)

    And "no energy loss" is not a "theoretical" concept. It may be "hypothetical" to suggest a scenario in which a machine operates with no energy loss but, as we all know, you don't get perpetual motion under any circs.
     
  9. Dec 8, 2009 #8

    Dale

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    You are certainly correct that the PE of the ship is increased by raising a ship in a lock. At the same time the PE of the water is decreased. This is no different in principle from hydroelectric power, or even old water wheels for mills.
     
  10. Dec 8, 2009 #9
    I think whats hes saying is this. lets take the lock out of the picture, as well as the water cycle. lets imagine a rubber ducky that's maxed out in weight with lead pellets inside, so that it barley barely floats. then lets put it inside a small fish bowl, and use a small pump to put in water in. after that we hang the ducky from a string. then using a water wheel and a small opening at bottom, get all the energy from he water back. we still have a lead filled rubber ducky with potential energy there. but since the duck was put there, and the amount of energy used to pump the water was the same as the energy when the water left. (saying we have a perfect system with little resistance) we somehow got free energy?
     
    Last edited: Dec 8, 2009
  11. Dec 8, 2009 #10
    I guess im still unable to get my question across.

    Let me put it another way. Floating is a result of differences in density, correct?

    So where does the energy that makes for example two liquids of difference density change place come from?

    Or another scenario: if you have a floating device anchored to the bottom of an empty reservoir and fill it with water, the floating device will gain PE as the water gets deeper..

    Can someone explain the source of this energy?

    Thank you
     
  12. Dec 8, 2009 #11

    Dale

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    Gravity.

    Consider a system of two liquids, one of density 2 kg/l and the other of density 1 kg/l, with 1 l of each liquid in a cylinder of 2 l volume and 4 m height. Now, consider the gravitational PE with the dense liquid on top vs the gravitational PE with the dense liquid on the bottom. With the dense liquid on top the PE (mgh) is:
    2 10 3 + 1 10 1 = 70 J

    With the dense liquid on bottom the PE is:
    2 10 1 + 1 10 3 = 50 J

    So gravitational PE is lost as the liquids exchange places and work can be done.

    What is the source of the energy for filling the reservoir? Whatever that source of energy it takes more of it to fill the reservoir with the same volume of liquid because the liquid must be raised further against gravity.
     
  13. Dec 8, 2009 #12

    sophiecentaur

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    Here's another way of looking at the problem.
    Imagine you had a very light, foam, boat. It will float almost entirely on top of the water. To make it go down to the depth of a 'normal' boat you would have to push down, displacing water and doing work (i.e. transferring energy) That is the amount of energy that is transferred when a boat sinks down or is buoyed up by rising water. It corresponds to lifting the volume displaced by a distance corresponding to the height of its centre of mass (or the centre of mass of the region of water displaced). I think that is the energy you are seeking to identify. It has to come from somewhere; it can come from someone operating a pump, when rain falls into a lake or for any of a thousand other possible mechanisms.
    Talking in terms of of density differences will explain where the forces come from but, as you know, work (i.e. energy) is the product of a force times a distance and it is energy that you after.
     
  14. Dec 8, 2009 #13
    So you could harvest potential energy from the rain by for example anchoring floating devices on the bottom of water reservoirs used for hydropower?

    By first emptying the reservoir (running it through the tunnel to the turbine) and transfort the original PE to electricity, and then anchoring the floating devices.

    After a certain amount of rain has fallen the floating devices will have gained PE and you could use generators to transform it to electricity when you let the floating devices go to the too.

    Would this make hydroenergy more effective or are we talking about very small amounts of energy?
     
  15. Dec 8, 2009 #14

    Dale

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    It would be very small, but yes. Essentially what would happen is that you would keep the rain water at a slightly higher elevation so it would have a slightly greater energy by mgh.
     
  16. Dec 8, 2009 #15

    mgb_phys

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    You can do something similar with waves - called Salter's ducks
     
  17. Dec 8, 2009 #16
    What abotu this a hydrogen Lighter then air blimp with wings you fill it with hydrogen let it get to 100,000 feet then use a pump onboard to reduce the hydrogen till the vehicle is heavier then air and glide with a 10-1 or even 20-1 glide ratio (due to buoyancy of the remaining hydrogen) and travel approx 360 miles or more i would say the energy gain is less then the energy used since moving the same object 363 miles at ground level would use far more power then the small electric pump used to reduce buoyancy so gravity would be a inexhaustible source of energy?
     
  18. Dec 8, 2009 #17
    Well if you are going to use the blimp to transport things it would be pretty heavy right?

    Wouldn't that require a LOT of hydrogen? Which would require a LOT of energy to compress?

    And an enormous hydrogenbaloon isn't too safe around heat? How about using helium?
     
  19. Dec 8, 2009 #18
    lol it was just for principle fine use helium lol and you wouldn't need to compress it all just say make it 100lbs lighter then air wait for it to raise then make it 100 lbs heavier then air dont read into it to much just understand the basic concept of what im trying to get across
     
  20. Dec 8, 2009 #19

    mgb_phys

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    So long as you want to go downwind, yes.
    I think long distance wind powered transport at sea level has been done - with the advantage that you don't need to go downwind and can move quite large loads.
     
  21. Dec 8, 2009 #20
    you can definantly go against the wind would just need to weight a little more meaning pump a little more helium out and you could even use the wind like a glider to gain altitude remember it turns from a blimp to a glider plus it still has the inertial weight lets say its a 20 ton blimp even tho its lighter then air it still cannot be nudged by wind just because it is lighter then the air the wind hitting it would deal with a 20 ton weight! so 100lbs less then boyant it will still have extreme PE even in a forward glide especially once it picks up speed it will be like a steam roller
     
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