# Unclear things about conservation of energy

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 ? B.

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.

• 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. – ACuriousMind Mar 28 '16 at 11:59
• @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 – user77648 Mar 28 '16 at 17:25