Question

Could anyone explain me in simple words what is being said in this topic from Feynman Lectures Vol.1 Chapter 4 topic 4-2 Gravitational Potential Energy (great book and author btw)...please refer to the book (pdf on net or a copy) to get what i'm asking.
i don't understand after it starts talking about perpetual motion and lifting and lowering of weights.please explain in simple words.
so i got confused at this point-
"Consider weight-lifting machines—machines which have the property that
they lift one weight by lowering another. Let us also make a hypothesis: that
there is no such thing as perpetual motion with these weight-lifting machines. (In
fact, that there is no perpetual motion at all is a general statement of the law of
conservation of energy.) We must be careful to define perpetual motion. First,
let us do it for weight-lifting machines. If, when we have lifted and lowered a lot
of weights and restored the machine to the original condition, we find that the
net result is to have lifted a weight, then we have a perpetual motion machine
because we can use that lifted weight to run something else."



umm like how exactly weight lifting machine lift one weight by lowering another.and what does that line mean that no perpetual motion at all is a general statement of the law of conservation of energy.im also confused by the last four lines and the whole topic that follows...can't seem to visualize what is being said...so if anyone can say the whole thing in this topic in simpler words or help me visualize it would be of great help.

im also a bit more confused by the next para-

"
A very simple weight-lifting machine is shown in Fig. 4-1. This machine lifts
weights three units “strong.” We place three units on one balance pan, and one
unit on the other. 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. Of course, we realize that with any actual lifting machine, we
must add a little extra to get it to run. This we disregard, temporarily. 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. "

why would you need to lift a little weight off the left pan?wouldn't the machine lift the right side(with one unit) up as soon as you placed 3 units on the left side?also what does the last four lines here mean again?how will you lift a weight of 3 by lowering a weight of one?how can one weight on one side make 3 weights on the other side go up?

Answer 1

First para explanation:

Perpetual motion machines are systems where the system is self contained and the motion of first thing generates motion for second thing and then this second thing generates motion for the first thing (The "thing" could be mechanisms of some sort ). You can add more mechanisms in between (In between the first and the last mechanism). In this way we get a machine which is self contained and just requires some initial energy input and the machine will run indefenitely, these are known as perpetual motion machines. Since nothing in the world is ideal, there will be always some loss of energy due to friction..etc, hence a perpetual motion machine is not possible as per the law of conservation of energy as the mechanism will continously loose energy and eventually it will stop. The example Sir Feynman is telling about the weight lifting machine where, there is lever action, (its like see-saw) so, when one weight lowers down, the other weight comes up.

Second para explanation:

As said above the weights are being lifted due to lever action, so one unit of weight is enough to lift the other three units of weight (or vice versa) since both the moments produced are almost equal we can say that in both ways the weights can be lifted (remember lever action).