Explain Energy (is it physical)?

[Baluncore]

Energy in Physics is like money in an economy, but ideal, without forgery and inflation.
Indeed the exchange rate between different currencies can be linked back to the fundamental economic currency that we call energy.

 

[Drakkith]

Energy in Physics is like money in an economy, but ideal, without forgery and inflation.
Indeed the exchange rate between different currencies can be linked back to the fundamental economic currency that we call energy.

Kind of like the quote in my signature!

 

[Baluncore]

Yes, kind of, but without the magic.
I can find no peer reviewed references to OmCheeto. What is it ? Did you invent it ?

 

[Drakkith]

Yes, kind of, but without the magic.
I can find no peer reviewed references to OmCheeto. What is it ? Did you invent it ?

You'll have to ask him.

 

[voko]

The important thing for all balance scales of all designs is that the total energy be minimum at the equilibrium position when the unknown mass is correctly measured.

As I said a few messages ago, this principle is far too broad. Many scales will have an equilibrium position when masses are not equal, and many (even though I am tempted to say 'all' here) scales will have an equilibrium position when one mass is too big to be measured, not to mention the infinitely many mechanical systems with some equilibrium which are useless for any practical purpose. You need some additional idea to measure masses.

And once it was pointed out that you were inventing some other principle you should have just said "oops" and moved on.

I do not see any "oops" in what I have said so far and I do not see any other principle. I have merely stated that in a particular scale design, in the context that scales may be interpreted as a contraption for measuring energies rather than forces - which you have agreed with - the equality of masses is equivalent to the equality of their potential energies. I am not claiming any generality of this, yet I find this observation important in the context "balance scales and the force vs energy dilemma".

 

[voko]

Of course it's possible to build something that way.

Do you have a reference (preferably with a picture or drawing) of a pracitcal balance scale built that way? Unless it has some other clever design features, I think it would be useless because the equilibrium position would be unstable.

Your statement was: "The central pivot is higher than the points where the pans hang from the beam."

A scale can have the pivot at the same level or even lower than the points where the pans are attached, and it can have a rigidly attached counter-weight below the pivot, so the entire rotating assembly is T-shaped. This will have stable equilibria within a range of mass deltas.

 

[jbriggs444]

Your statement was: "The central pivot is higher than the points where the pans hang from the beam."

A scale can have the pivot at the same level or even lower than the points where the pans are attached, and it can have a rigidly attached counter-weight below the pivot, so the entire rotating assembly is T-shaped. This will have stable equilibria within a range of mass deltas.

Let h be the height by which the pan attachment points are elevated above the pivot point. Let H be the distance that the counterweight hangs below the pivot point. Let m be the total mass in the two pans combined. Let M be the mass of the counterweight.

If mh > MH then such an arrangement will not be stable, even when the test masses in the two pans are identical.

 

[voko]

Let h be the height by which the pan attachment points are elevated above the pivot point. Let H be the distance that the counterweight hangs below the pivot point. Let m be the total mass in the two pans combined. Let M be the mass of the counterweight.

If mh > MH then such an arrangement will not be stable, even when the test masses in the two pans are identical.

Well, technically it will still have a stable equilibrium, but upside down :)

Practically, of course, the counter-weight and the masses will have to be within certain limits for the device to work as intended. That is true for any scale.

 

[OmCheeto]

You'll have to ask him.

I do believe I spend too much time on PF. I had a dream about this thread this morning. Odd thing is, I remember it clearly.

Voko and I were in physics lab, and the instructor placed a rock on the table. He asked us to describe the kinetic and potential energy of the rock, in relation to the table top.

My answers were zero, and, um, zero.

Voko's answer looked like the following: Click to see

 

[Enigman]

Yes, kind of, but without the magic.
I can find no peer reviewed references to OmCheeto. What is it ? Did you invent it ?

I was tempted to reply--"Its a meditating cheetah"--but as I have an extreme sense of propriety and a huge amount of self control I won't.
:p

 

[voko]

Voko's answer looked like the following: Click to see

Nah, you dreamed incorrectly :)

The kinetic and potential energies are not deduced from a Hamiltonian or a Lagrangian. They are required to formulate those things to begin with, so they are "given" pretty much like forces are "given" in the Newtonian formalism.

And writing equations with energies is not really more complex than with forces, to put it mildly. The success of the Lagrangian mechanics is a solid confirmation.

 

[AlephZero]

A scale can have the pivot at the same level or even lower than the points where the pans are attached, and it can have a rigidly attached counter-weight below the pivot, so the entire rotating assembly is T-shaped. This will have stable equilibria within a range of mass deltas.

Fair enough, I hadn't thought about using a counterweight to provide a moment to counteract the instability.

But with my mindset of working in aerospace, a counterweight is just an extra part in the device and extra mass, and those are two good reasons for not having one if you can make a design that works without it.

(Of course a torsion spring would be lighter than a counterweight, and provide the same functionality)

 

[voko]

But with my mindset of working in aerospace, a counterweight is just an extra part in the device and extra mass, and those are two good reasons for not having one if you can make a design that works without it.

A have seen a few "chemist" style scales, and they had a tall stand, a dial at the bottom, and the pointing needle (more like a spear) all the way from the fulcrum to the dial. I think it doubled as a counterweight. I cannot say with certainty, however, that the fulcrum was "low" in them, so they could have been a combination design.

 

[AlephZero]

But interesting as this digression on scales may be, it doesn't change the point I was trying to make in my first post, which is that all these designs are just as easy to understand using "forces and moments" as using energy. Either way, you need to understand the kinematics of the machine to make a mathematical model of it.

 

[voko]

But interesting as this digression on scales may be, it doesn't change the point I was trying to make in my first post, which is that all these designs are just as easy to understand using "forces and moments" as using energy. Either way, you need to understand the kinematics of the machine to make a mathematical model of it.

I do not disagree. My idea that balance scales can be tricky was a result of quite a few discussions with somebody having a hard time with some particular variety of balance scales.

 

[Dale]

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