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I About the Entropy of the Universe

  1. May 16, 2017 #1
    The Theory. Newton's Third Law of 1)Motion states: 'To every action there is an equal and opposite reaction'. ... The force exerted by the second body on the first body is called reaction. The action and reaction are equal and opposite.

    2)Second Law of Thermodynamics : In any cyclic process the
    entropy will either increase or remain the same. Entropy: a measure of the amount of energy which is unavailable to do work.
    If every action has equal and opposite reaction.... Than if I pushed a ball than it goes towards further... But it doesn't come back...... Here the law did not applied because of the total force applied by me is decrease by air resistance and friction..... And some force is used to do work.... Its confusing means why Newton 3rd Law and Thermodynamic 2nd law opposite to each other.... And universe entropy is increasing.... Why it increasing why ????? it doesn't obey Newton's 3rd......
  2. jcsd
  3. May 17, 2017 #2


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    Staff: Mentor

    That's not correct. The force you must apply to the ball (and the force the ball applies to you) is actually increased because of air resistance if you want to accelerate the ball at the same rate as a ball being accelerated in a vacuum.

    That's right, part of the total work is work done to move the air, part of it is in the form of energy lost to friction, and the rest is in the kinetic energy of the ball.

    Newton's third law and the second law of thermodynamics are not opposite of each other. Both are equally valid in all situations, including this one.
  4. May 17, 2017 #3
    But why universe entropy increases?
  5. Jun 3, 2017 #4
    My view is that entropy is what is called an emergent property. That is it only appears at a certain level of description. It is well known that at a fundamental level all processes are reversible. Think of a Feynman diagram of an interaction and the process can go both ways. Entropy is best visualized as a probability in that low entropy states are low probability of occurring so that nature tends to move towards states of high probability. Example – there is only one way to arrange a pack of cards in the standard order but many of any other order (about 10^68) so the cards are far more likely to be in any order bar the one they originally came as, excepting new packs. Boltzmann’s law states:

    S = k ln W,

    where S is the entropy and W is the number of configurations (k is Boltzmann’s constant). So any random process will tend to become more disordered.
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