# What Feynman meant in description of reversible machine and levers

In chapter four of the part one of the lectures, he mentions:

(..) 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.

and in the picture (is the fulcrum a bit of placed w.r.t the contex ?)

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.

I can't make any sense out of this.

If the example Feynman gave was "perfectly reversible", then you could move the balance up and down without doing any work. In order to overcome (even tiny amounts of) friction, you actually need to do a little bit of work. This can be done by adding or removing a little bit of weight to one of the pans of the scale.

After a cycle of moving the scale first one way, and then the other, the "state" of the scale is the same as it was before. In a perfectly reversible system, the total work done would be zero. But if you had to lift a little bit of weight to the top scale in order to tip it, then move it to the other scale to tip it back, you would have done a net amount of work on the system.

That makes this system (and any other "real world" system) irreversible.

Incidentally, the fulcrum is offset because, with 3x the weight on the left of the scale, you get "balance" when the lever on the right is 3x the lever on the left. One could have done the same example with a symmetrical scale with the same weight on each side instead.

• 1) Feynman is talking about the real world, so effort is needed; 2) the 1W moves up, and then back down (to the same height where it started) 3) all kinds of friction: air and the hinge / pivot, mostly; 4) Scale is balanced (or close to it), but if center of mass is at the pivot (rather than below it) then "balanced" means "no restoring torque" results when the scale is moved slightly (unlike a normal scale); 5) I think he is talking about adding a very small "helper weight" to move the scales around. It could be a "chip off the big weight" but that doesn't change the argument. – Floris Mar 29 '16 at 17:46
• 1- the energy goes to heating the contents of the ideal box ("heat is work and work is heat"); 2- the important point is just that any machine has some dissipation (friction etc) turning work into heat irreversibly (as opposed to ideal Carnot cycle which is fully reversible). 3- if you consider not 1 and 3 but 1 million and 3 millions grains of sand, you can't tell the difference between a "helper weight" and a chip off the nigh weight. Was it grain of sand #1,000,000 or #1,000,001? No real difference. – Floris Mar 30 '16 at 10:36
• For anything to "stay balanced" means that it requires a force to move away from the point of "balance". If there is some static friction, then "balance" can happen over a range of positions. The less friction there is, the more you rely on the shape of the gravitational potential. If you have a marble at the bottom of a round bowl it stays there. If your marble is sticky, it will stay some way up the side of the bowl as well... If that doesn't clear it up you need a face to face conversation - do you have a physics teacher? – Floris Mar 30 '16 at 11:27
• If you are reading Feynman at 14 I tip my hat at you! Curiosity beyond the material taught in class is the way to become a master of the subject - unfortunately it may also lead to friction with your teachers and class mates. Sorry to hear even your private teacher is not meeting your need. Come back here and ask more questions! – Floris Mar 30 '16 at 11:51
• Imagine a cylinder with frictionless piston. Push on the piston and you do work on the gas. Pressure and temperature go up. Reversible engine: gas stays hot until you release the piston - it can push the piston back to the original position, "giving back" all the work you did. Irreversible form: while the gas is compressed, some heat flows into the cylinder. Gas cools down and it can no longer push the cylinder back with the same force. Work was irreversibly converted to heat and entropy has increased. Does that help? – Floris Apr 2 '16 at 3:04

Feynmann often liked to deal with realistic real world examples instead of idealised ones, to illustrate how physical principles applied in the real world. In this case I suspect that what he is getting at is that a real world hinge/balance isn't frictionless, located at a mathematical point and made of perfectly rigid materials. In the real world, you can actually balance a mass of 3 Kg against a mass of 1 Kg located three times further from the fulcrum over a small range of positions and have it be stable. To make the balance reliably tip, you need to have a slight imbalance in the weights (or distances) as due to friction and material deformation (the arms of the balance bending for example), the situation is not perfectly reversible.