The incredible Backwards Walking Rocking Chair

Not sure I understand the question - so here's my answer - possibly to something else.

Two hangings, each from one string, should produce two lines that intersect at the CofG. In practise small errors (due to air density not being uniform, stiffness of the string, gravitational non-uniformities, flexibility of the rocking chair, etc.) will result in the two lines missing each-other by a finite amount.

Two hangings, one from two spaced strings and one from one string should result in a plane being intersected by a line. The positional error will possibly be greater, but undetected.

Three hangings each from one string will result in three lines that miss each other by a small amount.

In each case, if everything was ideal and the world was Newtonian, the differences and the error would tend to zero (or infinitessimal) - I suppose that is what your teacher meant. In the physical world as currently partially understood, Heisenberg would ensure that the error was finite.

I hope that was at least interesting, if not helpful.

Fyz

Put me right out for a nap!

OK, one-more-time. Three hangings, one-at-a-time. One hanging gives you a line on which the CG exists. The next hanging, gives you a plane on which the CG exists, and, finally, the third hanging gives you the point on the plane were the CG is located. You can "guesstimate" many simple shapes - Weren't you happy the day you realized the CG of a donut was at the center where there was no donut stuff! Whee! Your parents were so proud, that they sent me the picture of you wetting yourself (and the negative)!

Unless you're some sort of CG savant, I'd suggest hanging it three times. And if there's an error I doubt it will be large enough to interfere with the space/time continuum.

Have you ever tried to bundle detritus?! You'll have about as much luck doing that as you will finding the center of gravity on a complex shape with complex density.

All Iz noze is they told us we had ta hang a car by 3 different points ta geet the CofG.

And while were at it, isn't "CofG" a soft drink from Bulgaria?

Indeed, I did precisely that earlier this evening when putting the wheely-bins out for collection. I always wrap the kitchen waste in a layer of newspaper before placing it in the appropriate bin - so far this has stopped the bin from stinking and saved me the trouble of hosing it out.

Dey wus rong. The hanging method knows nothing about the complexity of the shape.

It may be that they read it somewhere, but forgot the details of what they read.
Hanging from a single string each time requires just two hangings.
But you will need three hangings if you hang the object from two spaced strings each time - and for reasons of safety you would probably want to use two suspension points for a car (you wouldn't want such a heavy object to rotate too freely around a single suspension point).
I suggest you build a small model of what can happen with the various hangings, and trust your experience rather than third-hand (or possibly higher!!) authorities.

Fyz

Doesn't everybody have three hands?! What?! What are you not telling me here?!

Sorry, forgot to say - the higher primates generally have four hands, as have many rodents (but not, SFIK, Nutkin). Again SFIK, only the best-looking humans have three hands (don't ask - you really don't want to know).

Nutkin is going to dig up a coprolite and chuck it at somebody soon. With one of his paws. Do many hands make light work ?

κοπρος/kopros λιθος/lithos...

And what's wrong with Joe Satriani?

He's vey cool , but Lenin shouldn't have hooked up with the Japanese woman.

"Listening to ice picks can damage your hearing."

Yeah, but she had them doing performance-art before it was cool to do performance-art.

You're probably right.

I switched to Canadian time and nobody can see it .

It's 6:23 am really.

It's always AM in Canada. That's the only way they can get rid of all the bacon they produce.

How fast does a truck get down to the Sta..

Oh never mind.

I'm feeling light-headed. This thread is plumbing new depths.

Electrickery makes light work......