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Energy question

  1. Dec 23, 2005 #1
    Hello all,

    Its my first thread here!
    I'll start things off with a (simple?) question.
    Suppose a block is suspended on a surface of water. It comprises potential energy quite obviously. As the block desends down the water level, it loses PE. According to the rules of conservation of energy, if the block is losing PE, surely the water is gaining PE. My question is how? It's not like the water level has changed! Or is it gaining PE in terms of internal energy?
  2. jcsd
  3. Dec 23, 2005 #2


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    The water level is not rising???

    Then why does when I imerse my spoon in my fully loaded bol of soup, it spils everywhere?
  4. Dec 23, 2005 #3
    um what? I dont get you...could you pls, kindly, clarify your question?
  5. Dec 23, 2005 #4


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    Let me first describe the situation I'm picturing to see if it corresponds to what you had in mind. So you have a block of something (say, lead) and it's being suspended (by say, a rope) right above the surface of some body of water. Then, we cut the rope and the block begins to sink. The first point is that it will only sink if its density is greater than that of water. Otherwise, it will float on the surface, partially submerged (see Archimedes' principle).

    Quasar's point is that, as soon as it enters the water, it will be displacing some of the liquid and the water level will rise. This is true and it will lead to a rise in the water's potential energy, but it will only be a small change and will only occur until the block is completely submerged. After that, the water level will remain constant as the block falls.

    Now, what happens during the fall? Well, if there were no water, the block would fall freely, losing potential energy and gaining kinetic energy (energy of motion). In the limit of a very dense mass, this is what will happen in the water as well -- there would be effectively no energy transfer between the block and the water. However, for most objects, there would be a non-negligible amount of drag as they fell through the water. This means that the water molecules would push back on the block, a force opposite to gravity. This force would do negative work (i.e. the block would do work on the water) and there would be a transfer of energy from block to water.

    What form does the energy take when it's passed to the water? Well, it depends. Some of it might be transferred to thermal energy; that is, it might heat the water. An increase in temperature means that the water molecules would be moving more quickly on average, so thermal energy can be thought of as a form of kinetic energy. Another thing that can happen is that the water can gain turbulent motion. This means that, instead of an increase in the average motion of the molecules, it can produce a bulk motion of the molecules. The exact details of the energy transfer can become complicated and it will depend on other things, like the temperature of the water and the shape of the block. Hopefully you get the idea, though.
  6. Dec 23, 2005 #5


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    I couldn't explain any better than Space Tiger already has. However I fear you may question this phenonema in theory and go away discontent. So to avert such feelings from taking you over, may I please suggest you to collect a measuring container (I feel confident you possess any type of liquid container with measurable readings etched or embedded on the side) and a considerably big (so long it fits well in the container) and heavy object. Fill the container up to a preferable reading (may I suggest a middle mark reading) and slowly emerge your chosen object on the container and wait until it hits the bottom.

    Is the water level the same or has it changed with the object present? Record readings before and after if it helps
    Last edited: Dec 23, 2005
  7. Dec 24, 2005 #6
    Thank you all for the responses! :)
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