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B Irreversible machines

  1. Jul 26, 2017 #1
    Hi all,

    I still have "a many times discussed" problem to understand Feymann's issue about irreversible/reversible machines example.

    Here is the part from the original text.
    Here is my questions:

    1. Here Feymann talks about the situation when you are not applying any force to rise or lower the weight. Your system is closed (isolated) for any applying force, you only supply the weights for the system. Lets talk about irreversible machine like Feymann says
    Here in the case of irreversible machine we don't supply any energy to rise the weight. But if we do not supply the energy and the machine returns to the initial state and in the end the weight is raised we have the perpetual movement.

    Take a look at the picture of the real irreversible machine:

    feynman irreversible machinea.gif

    here the link which could help: https://phet.colorado.edu/sims/html/balancing-act/latest/balancing-act_en.html
    feynman irreversible machine3.jpg

    The first picture is perfectly balanced condition with baskets for weights. Lets neglect the weights of baskets and weight of lever and friction of fulcrum. Now, we put the weights into the baskets and then we have 5kg is lifted and 10 kg is lowered. Now we remove the weights and the scales back to the balanced condition. So we didnt apply any energy to lift the weight and the system back to the initial condition and in the end we succeeded to arise the weight that can do something else (as Feynmann says)
    . So we have perpetual motion (according Feynman definition) here? If we do it again we will lift another 5kg and again 5kg.....

    Attached Files:

  2. jcsd
  3. Jul 27, 2017 #2
    I think with that last statement you switched back from irreversible to reversible machine.
  4. Jul 27, 2017 #3
    Why do you think so? I neglect it to return this machine to initial horizontal state. But anyway if you place the weights of 5kg and 10kg in the places as depicted in the figure it will always lift the 5kg and lower the 10kg-irreversible behavior.
  5. Jul 27, 2017 #4
    Well, the inability of returning it to the original state without friction losses is the very point of Feynman; it is the difference between a real machine (irreversible) and an idealized one (reversible).
    In this universe, you can only build real machines.
  6. Jul 27, 2017 #5
    I think I know where I was wrong. Forget all the losses. I forgot about the 10kg. After the action (Figure 1 case2) 10kg is below the balanced state. To back it to the balanced state (horizontal state) we should apply work against the gravitational energy to put it back. There is no such a problem with 5kg. 5kg is above the balanced state (horizontal state). You see, to back this machine to the initial state we should do work to return 10kg to the balanced state, i.e. we should interfere in the isolated system (to let Bruce bring his blocks). But it isn't the case of perpetual mobile system. Perpetual mobile system should be isolated (closed) from interference from outside and give free energy forever. So this system can be work as perpetual mobile. But in the case of Feynmann example to do perpetual mobile we should use reversible machine to get back the weights to the balanced state. But another condition for the perpetual motion we need to lift our weight with irreversible machine higher than reversible machine can in order to get free energy.
    Last edited: Jul 27, 2017
  7. Jul 27, 2017 #6
    I can't quite tell whether you drew the correct conclusion or not, but the bottom line of all of this is:

    - Energy is conserved at all times
    - All real machines incur irreversible losses
  8. Jul 28, 2017 #7
    You are absolutely right. Finally I understood the point I couldnt understand. why we can't build perpetual mobile with irreversible machine. Now I understand. We need to do work to get back 10kg to the initial state. This is it. I understand why reversible machines you can't build. Because in order to incline the lever to one of the sides you have to do some work "even infinitesimal", that's why Feymann called it "almost reversible" (even if you are neglecting friction). But if you are not neglecting friction you should do a bit more work to incline the lever to one of the sides.
  9. Jul 28, 2017 #8


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    Staff: Mentor

    Ha! If by "we", you mean *me*, sure - I'm not going to do that. But *you* are going to expend a lot of energy lifting those weights over and over to reset the machine.

    Anyway, we don't discuss perpetual motion here, even to debunk it (though I just did...), so this thread is closed.
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