Re: Stone Shifting 2 | Forum — The Modern Antiquarian

Steve Gray Aug 28, 2003 23:41:17

Replying to baza

Hi Baz

> I presume that we`d be going for the lowest figure.

Nope, were going for the biggy. A full Trilithon with 40 ton uprights.

> I don`t know how deep the stones were set into the ground.

Apparently it varied, but about 8 feet seems a typical figure.

> By depth, do you mean width? The width of the stone dragging on the tower? Or is that irrelevant once you know the coefficient of friction?

OK, I had to decide whether to refer to the stone as height and width or length and depth, depending on whether you consider it on its side or erect. Because I started calling the height of the tower "height", I decided that I would consider the stone on its side and refer to length and depth to avoid having two different things called "height". That leaves "width" to describe the remaining dimension (the one that's in contact with the tower). Length and depth are the only two figures that effect the moment of inertia (for a stone of uniform dimensions). The width is irrelevant and also does not affect the friction to any significant degree. The coefficient of friction depends only on the nature of the two surfaces regardless of the area of contact. The frictional force (in any given situation) is just the weight acting onto the surface times the coefficient of friction (end of today's course in A-level mechanics).

> Can you tell me what the offset variable relates to, please? I don`t understand that one.

Yes, it's the overhang of the centre of the block from the edge of the tower. I started calling it overhang and refered to it by this name in several TMA postings, but then I changed it to "Offset" just because "Overhang" was too long for the screen space I had allocated. Since my main concern was to finish the model and get it posted I didn't want to spend time resizing all the boxes. Sorry if it wasn't clear. I did think about putting dimension arrows on the picture whenever the cursor moves over a text box. So if you put the mouse over "Height" a dimension arrow would appear next to the tower showing which dimension was height on the picture. Unfortunately this idea was also subject to the time guillotine.
It might make it into the next release, but my main concern is to correctly model the hole.

baza Aug 29, 2003 12:27:19

Replying to Steve Gray

Thanks for the explanation of my queries on the model.

>Nope, were going for the biggy. A full Trilithon with 40 ton uprights.

So, we are talking about the erection of one of the inner horseshoe of trilithons at Stonehenge, utilising the stones used by the BBC in their experiment.

Presumably, that would entail the `rowing` of three stones to an 8 feet deep hole (both uprights would have to go into the same hole). The raising of the two uprights on their platforms to a sweet height to tip into the hole, followed by the raising of a lintel, twenty feet above the ground on its tower, for lateral movement onto the uprights.

I can foresee a few serious problems!

Do we know the dimensions of the BBC stones?

baz

Steve Gray Aug 29, 2003 18:27:07

Replying to Steve Gray

Just to set the record straight (although nobody seems to have made an issue of it) I said that "length and depth are the only two things that affect the moment of inertia". What a load of cobblers! Of course weight (or to be more precise "mass") has rather a lot to do with it too, but since weight is also the driving force it cancels out from the equations, leaving only length and depth as the critical values. So a block stone behaves in exactly the same way as a block of balsa wood of the same dimensions - except when it lands on your toe.