1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help with Conservation of Energy Principle

  1. Aug 17, 2005 #1


    User Avatar


    I am not able to reason my way through conservation of energy
    when I consider the following scenario..

    Suppose you are holding up a weight and there is no movement of the
    weight. According to theory this means no work is done on the weight
    since net forces on it cancel and there is no movement. However it takes effort to hold it up and eventually you get tired which means that you are using up chemical energy to hold up the weight. So if no work is done which
    means there is no energy transfer and yet chemical energy is being
    used up to hold the weight (since you are getting tired)...where
    does this chemical energy transfer to? It's not surely vanishing
    or dissappearing as heat...

    my imagination rather than reason says, its supplying the brick's
    potential energy which is being 'sucked' away by the force exerted on
    it by the earth..so energy being transferred to earth??...this seems
    totally wrong but I can't seem to reason here...please help

  2. jcsd
  3. Aug 17, 2005 #2


    User Avatar
    Science Advisor

    Suppose the block is sitting on a table, or on the earth itself, rather than in your hand. Would you argue the same way? How long would it take for all of the block's potential energy to be "sucked away" in that situation? As long as the block, in your hand or on a table, stays at the same height, it has lost no potential energy to the earth or any where else. In order to talk about "conservation of energy", you have to take into account all forces- there must be no "external forces". In the example you give,you have to account for forces internal to the muscles themselves as well as the "friction" there due to the fact that your muscles are not perfectly efficient- they lose energy to heat and vibration. (If you hold a heavy block at arm's length for any length of time, you know what I mean by "vibration"!)
  4. Aug 17, 2005 #3
    Doesnt gravity's force on the earth account for the heat emitted from the earth's core? The pressure on the iron raises its temperature as I understand it. I know Blackbody radiation converts the iron's mass slowly into energy, but I heard that Blackbody radiation is slowly emitted only at high temperatures, this is the cause of iron turning red with high thermal energy. But say is the mass wasnt enough to cause it to emit blackbody radiation, it would still make it warm or hot, just not melt...right?

    Another thing.. what if say a meteor hit the earth? It wasnt lifted to give it potential energy, but it was suddenly given KE just by getting close to the earth. And then when it hits the earth, then the gravitational force INCREASES! It gave an object KE out of nothing, and in turn it exerts more force then it did then.
    (I dont know if that made any sense...)
    Last edited: Aug 17, 2005
  5. Aug 18, 2005 #4
    I dont understand your argument about the Black body radiation. when iron is heated it looks red bcoz it is emitting most photonscorresponding to that wavelength.
    about the meteor; the meteor as it travels through space it already has KE; nobody gives it any KE. when it is very near the earth it may get attracted to the earth by its gravity and thereby collide with it. U dont have to lift it to give PE. In the stellar scales; u see gravitational PE that has a different formulae.
    and about the heat in the earth's core; whose gravitational force on what do u want to imply?
  6. Aug 18, 2005 #5


    User Avatar

    Staff: Mentor

    Anyway, to put a finer point on it:
    It is being dissipated as heat.
  7. Aug 18, 2005 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Since the brick does not move, there is no work done on the brick and thus no mechanical energy transferred to the brick. But to maintain the tension in your muscles that creates the force supporting the brick, your muscle fibers continually contract and relax: that requires energy. (Since there is no net motion of your muscles, that chemical energy ends up as thermal energy.)

    Compare this to just resting your hand on the table, palm up, and laying the brick on top. In this case your muscles no longer have to maintain tension to create the force supporting the brick, thus no additional energy is needed.
  8. Sep 19, 2005 #7


    User Avatar

    Thankyou for resolving this. So the brick does not change its energy 'state'. The transfer of energy is simply from chemical to thermal. This makes sense now.
  9. Sep 19, 2005 #8
    In physics , equation for work done by a force is only applied for external forces , that is the scenario external to the body or system in consideration is taken into account. In the cases of holding the weight in air or pushing the wall , there is no net displacement as viewed from outside and hence work done is zero. But if we consider the muscles of the man who is holding the weight , these muscles are not rigid rods , these muscles need to do work to act as a straight rod for a transient time . The muscle fibres in these muscles contract and expand to make this sure and hence work is done in contraction and expansion , which leads to lowering of the internal energy of the man and it keeps getting difficult for man to maintain the straight rod act.

  10. Sep 19, 2005 #9


    User Avatar

    The Earth was formed by the collision of rocks and dust that were present at the formation of the solar system. Those materials collided because they were attracted to each other by gravity. At the collision kinetic energy was transformed into heat. Since heat only can be lost in vacuum by radiation, most of the original heat is still here and has fused the material that composes the core.
    You are right. Gravity is responsible for the heat in Earth's core.
  11. Sep 19, 2005 #10
    The largest component of energy that keeps the earth's core molten is the rotation of the earth within it's own gravitational field. Tidal like force cause the core material to move generating huge frictional heat. That heat is supplemented by heat of compression from the full mass of the earth.

    If the earth didn't rotate it would have already cooled off to a solid core a billion years or so ago.
  12. Sep 19, 2005 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    To make sure this thread stays on topic, the OP requres NO explanation about the earth's molten core. So please do not hijack the thread into something else or I will be forced to perform some surgery onto it to move the relevant postings out of here. Trust me, the end result will not be pretty.

    Some of you are MORE than welcome to start a new thread in the Earth science section.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook