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The bluestone debate
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Steve Gray
Steve Gray
931 posts

Re: The bluestone debate
Nov 19, 2008, 15:34
tonyh wrote:

You don't need wheels for this effect. Dragging just about anything that has weight will give you the same result. A stone on a sledge will be slowed by varying ground conditions a ripple will run down the team of Oxen. The longer the Train the worse the effect..

Tony


You seem to be fond of taking one observation, extrapolating it to a different situation and then asserting as fact something for which you have no scienctific justification.

If you consider one pulling unit (a pair of oxen, or a team, or whatever) attached by a single elastic rope to an object and strong enough on its own to overcome the limiting friction of that object then yes, there is an effect called resonance that will produce what you term the "push pull" effect. It's similar to an undamped spring. The resonance is further exacerbated by the fact that moving friction is somewhat less than limiting friciton.

That's your one observation and it's quite correct at that level.

However, there are several factors in my suggestion that you don't seem to appreciate.

1. There is considerable friction, which acts as a damping force, just like the old friction shock absorbers on leaf-spring cars.

2. There is considerable mass, so the forces required to alter the momentum of the system are very much larger.

3. I was talking about lots of individual pairs of animals each with its own rope attached to the stone. The force in any one rope is insignificant compared to the weight of the stone, so individually a rope cannot significantly influence the inertia of the stone and hence will not individually induce resonance in the whole system.

4. The combined effect of all the ropes will be a relatively constant force despite individual fluctuations.

5. Resonance will occur most readily if the relative frequencies of the puller and the object are similar. The resonant frequency of a system is inversely proportional to mass. The relative frequencies of an ox on a rope and a 40 ton stone on the same rope are so different that one will not tend to induce resonance in the other.


If you doubt this try it. Get a group of say 10 friends each with a length of stretchy rope and tie them to a car with the handbrake partly on (to simulate dragging friction). If some of you begin to pull you will merely stretch your own rope and not achieve much. As the rest start to pull you will reach a point at which the car will begin to move forwards.

At this point the combined pulling force has just overcome limiting friction, so the car will begin to move forward slowly. All you need to do now to keep it moving is to pull steadily and the car will follow you matching your combined walking speed.

Nobody can go faster than this speed because they don't have the strength to speed the car by very much up on their own. Even if one of you starts messing about alternately tugging and stopping it won't have much effect for the same reason.

On the subject of ropes, Nigel mentioned the Norwegian rope maker. My recollection of her rope was that it was about 5 cm diameter and had a breaking strain of around 4 tons.
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