I have started reading Feynman Lectures in Physics, First Volume, with absolutely no background in physics [*]. In the fourth chapter, conservation of energy, I am having difficulty understanding these things:-

A. Feynman defines reversible motion as:

Ideal machines, although they do not exist, do not require anything extra. A machine that we actually use can be, in a sense, almost reversible: that is, if it will lift the weight of three by lowering a weight of one, then it will also lift nearly the weight of one the same amount by lowering the weight of three.

My problem here: 1. Why actually building a reversible machine is not possible here ? (Please clarify unintroduced jargon) or Why the word "nearly" ?

  1. Would not the two side balance in the following picture ?

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However, in order to get it actually to work, we must lift a little weight off the left pan. On the other hand, we could lift a one-unit weight by lowering the three-unit weight, if we cheat a little by lifting a little weight off the other pan.

  1. i. Is it for reversible machines or our day life almost reversible machines ? ii. Instead of lifting a weight, would not adding a piece of weight work ?

  2. What is lifting ? In general case we add, say, half block on the three block's side -it would (assuming it is balanced earlier) lift the one block higher. But (my intuition is fallible) intuition is saying that doing the reverse will not lift the three block as high. Am I right ?

Does it has any connection with reversibility of it ?

  1. After concluding

one pound falls a certain distance in operating a reversible machine; then the machine can lift pp pounds this distance divided by pp.

How does Feynman conclude

If we take all the weights and multiply them by the heights at which they are now, above the floor, let the machine operate, and then multiply all the weights by all the heights again, there will be no change. (We have to generalize the example where we moved only one weight to the case where when we lower one we lift several different ones—but that is easy.) We call the sum of the weights times the heights gravitational potential energy.

  • 3
    $\begingroup$ This seems to be a very wordy question that's answered by just one word: Friction. All real-life constructions will have friction, dissipating the energy over time. $\endgroup$
    – ACuriousMind
    Mar 28, 2016 at 11:59
  • $\begingroup$ @ACuriousMind I do not have any background in physics, and any source except Feynman (I have few) is promoting plug-n-chug, so please describe how friction is answering the questions $\endgroup$
    – user77648
    Mar 28, 2016 at 17:25

1 Answer 1

  1. See 2.

  2. Because of loss due to friction: air friction, the triangle's point won't be perfectly pointy but rounded, the bar will flex when you start moving it (accelerating it) then stop moving it (decelerating it), producing heat, etc.

  3. Since the system is in equilibrium, it won't move on its own. To get it moving, you have to add some energy that becomes kinetic energy of the beam. Adding weight isn't really the opposite of lifting weight. By lifting a weight, you're applying force, but the mass stays the same. The opposite of lifting a weight would be pushing on the other side. That would work too.

  4. By lifting, he means grabbing it and pulling. Not enough to take it completely off the platform. Just enough to set it in motion. Have you ever seen kids on a see-saw? When one pushes against the ground with their legs, that's like Feynman's lifting.

  5. If one pound falls a distance d_1, then pp pounds will fall a distance d_pp = d_1 / pp. The distance moved isn't constant, it depends on the weight. But distance times weight is constant: d_1 * 1 = d_pp * pp. In other words, if you have a weight w1 and you move it through a distance d_w1, then how much can you move a weight w2 ? w1 * d_w1 / w2. It's useful to give a name to the parts of that equation that are specific to a single weight: w1 * d_w1.


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