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Question about mechanical energy and lowering a block

  1. Nov 18, 2014 #1
    When one lifts a block off of the ground and holds it in the air, the work done by one's lifting force is equal to the change in total mechanical energy of the block/earth system. Since the mechanical energy is KE+PE and since KE doesn't change during the motion, the work done by the lifting force is equal to a change in PE. This is obvious and is easy to visualize from an energy transaction perspective: the biochemical energy in the arm muscles gets converted into potential energy.

    However, when one lowers a block from a shelf and places it on the floor, the work done by the lowering force is equal to the change in total mechanical energy and that change is negative since the KE is constant and the PE goes down. Since the mechanical energy of the block/earth system went down and since all energy is the universe is constant, where did that energy go? If the block/earth system's PE went down, something else's energy must have gone up. Did the thermal energy in the bones joints of the person lowering the object go up? Did elastic energy in the atoms of the person's body go up due to the weight of the object? Clearly, the person's biochemical energy did not get replenished by the drop in potential energy (which would make no sense), but what form of energy did go up?
     
  2. jcsd
  3. Nov 18, 2014 #2
    First, forget about the human body, and instead imagine the task is being done by a machine - a crane. The weight is supended by the cable, which goes through the upper pulley of the crane and then is wound around the cable drum. When the drum is locked, the weight is suspended motionless in the air. To lower the weight, the drum is unlocked but the brake is applied, and as the weight descends, the brake heats up and dissipates heat to the surrounding air. That's where the potential energy goes.

    Now in the case of the human body replacing the crane: if you let the cable slip through your hands you will feel the frictional heating. The less friction, the less the energy is dissipated (and the greater the KE due to velocity at the time the weight hits the ground). If you let the weight down without slipping -- say by hand-over-handing it down -- the story gets more complicated than classical mechanics. You need biology / physiology to answer the question in that case.
     
  4. Nov 18, 2014 #3
    gmax137 gave a very good answer until this point:
    No, you really do not need biology / physiology to answer the question; the answer does not change. The energy is still dissipated as heat.
     
  5. Nov 24, 2014 #4
    Thanks for both of your answers. I think I agree more with gmax137. I think that the energy is most likely dissipated as heat (potential friction in the elbow joints), but I don't know enough about the body to be certain. But why couldn't there be other bodily mechanisms that dissipate the energy?

    At the end of the day, what I wanted to get at is that the body is indeed absorbing the energy in some form, just like the crane's brake friction with the rope keeps the potential energy from being into kinetic energy.
     
  6. Nov 24, 2014 #5

    CWatters

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    Some parts of our body can store mechanical energy - for example when running the tendons in your legs act like springs and can store some energy, but only briefly.

    As far as I'm aware the processes that make our muscles work aren't reversible. eg Your muscles use oxygen but can't generate oxygen. We haven't evolved regenerative braking yet.
     
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