Wednesday, August 6, 2008

An analysis of the forces required to drag sheep over various surfaces

ResearchBlogging.orgI'll be honest, I'm mostly blagging this piece from Applied Ergonomics (Volume 33, Number 6, November 2002) because it has such a terrific title. It's the title that got the paper an Ig Nobel (see this story in WIRED) a while ago, suggesting that the Ig Nobel processes at least sometimes don't get beyond the title.

The motivation for the study was related to occupational health. Sheep shearing involves some dragging work, and the shearers are prone to back injuries plausibly related to the dragging. Here's the abstract:
Some occupational health and safety hazards associated with sheep shearing
are related to shearing shed design. One aspect is the floor of the catching
pen, from which sheep are caught and dragged to the shearing workstation.
Floors can be constructed from various materials, and may be level or gently
sloping. An experiment was conducted using eight experienced shearers as
participants to measure the force exerted by a shearer when dragging a
sheep. Results showed that significant changes in mean dragging force
occurred with changes in both surface texture and slope. The mean dragging
forces for different floor textures and slopes ranged from 359 N (36.6 kg)
to 423N (43.2 kg), and were close to the maximum acceptable limits for
pulling forces for the most capable of males. The best floor tested was a
floor sloped at 1:10 constructed of timber battens oriented parallel to the
path of the drag, which resulted in a mean dragging force 63.6N (15%) lower
than the worst combination.
The rest of the paper is hardly exciting - they got some shearers and some sheep, and interchangeable panel to experiment with different surfaces, a force plate, did some biophysical modeling, and came to the conclusion at the end of the abstract. The 'change' that came with changes in slope was the anticipated 'finding' that its easier to drag a sheep down hill. As with
much applied work, doing the sums meant including some simplifying assumptions, my favourite being:
The best available estimate of the force being applied by the hands of the shearer is predicated on the assumption that the sheep were inelastic and moving at constant velocity.
This simplification leads to a caveat at the end, since, given that it takes more oomph to get the sheep moving in the first place, and those sudden bursts of effort is likely a key culprit in wear and tear on the back:
Another issue is that the initial force required to begin motion exceeds the steady-state dragging force. Calculation of this initial force from force plate measurements was infeasible because estimates of acceleration and elasticity of the sheep would be required.
Acceleration seems like the sort of thing that you could measure easily enough, maybe even from video footage, but I'd guess that rigorous assessment of the elasticity of sheep would demand a study in its own right.

Harvey, J. (2002). An analysis of the forces required to drag sheep over various surfaces. Applied Ergonomics, 33(6), 523-531. DOI: 10.1016/S0003-6870(02)00071-6

3 comments:

Anonymous said...

Good to know...
-John

paul coombes said...

I am confused when you quote "The best available estimate of the force being applied by the hands of the shearer is predicated on the assumption that the sheep were inelastic and moving at constant velocity" which seems at odds with the abstract of "measure the force exerted by a shearer". They then go on to say "Calculation of this initial force". so were they measuring the force or calculating it? If they were calculating why weren't they measuring?
Despite this I do like concepts of elastic and inelastic sheep.

Doctor Spurt said...

It's both measured and calculated: measured on the floor plate, calculated via a model for the stresses on the shearer, who (among other things) isn't pulling parallel to the plate, or perpendicular to gravity.

(If a pure physicist had done this, to slightly adapt the old joke, the paper would have started with "Consider a spherical sheep...".)