sloshing water: effect on energy/effort

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sloshing water: effect on energy/effort

Postby wildwanderer » Mon 19 Aug, 2019 9:04 pm

This is a bit of a side diversion to the below soft water bottle topic. Possibly belongs in techno babble. :mrgreen:

I commonly use 2 x 1 L PET bottles. I couldn’t find two cheap 1L ones last time i visited the super market, so grabbed 2x 1.5L.
Typically, I fill with around 500ml each (and occasionally 1L each if it’s a big day with no resup points)

On my osprey pack I have these in the side pockets on a angle. With 500ml of water and 1.5L bottles I very much noticed the slosh effect.. the water bouncing and water weight shifting back and forwards as I moved.

And I wonder if this sloshing is causing me to use up more energy... either through my muscles needing to stabilise the load more or because the water sloshing backwards is energy heading in the reverse of my walking direction..

Thoughts?

(its not something I’m overly concerned about... more scientifically curious)
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Re: sloshing water: effect on energy/effort

Postby crollsurf » Mon 19 Aug, 2019 10:20 pm

Scientifically speaking I do like to keep my pack balanced so swap around my bottles because I favour my left when reaching for water, but other than that...

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Re: sloshing water: effect on energy/effort

Postby slparker » Tue 20 Aug, 2019 4:33 pm

I am not a physicist but there is no more energy required to carry a 1kg/1l sloshing bottle up an incline, or when walking, than any other 1kg weight.

that said, the postural muscles will need to do more work to accommodate the shifting load as the weight moves across the lateral plane.

So no more energy is required to bear the static weight but there is more physiological cost to accommodate balancing the shifting weight.

I don't know how big a deal it is but if you're worried carrying the load centrally (i.e. in the vertical plane) will virtually negate this effect. hard to reach a water bottle carried in your pack though...
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Re: sloshing water: effect on energy/effort

Postby skibug » Mon 26 Aug, 2019 12:40 pm

"...And I wonder if this sloshing is causing me to use up more energy... either through my muscles needing to stabilise the load more or because the water sloshing backwards is energy heading in the reverse of my walking direction.."

1. The sloshing water will be constantly changing the position of the Centre Of Mass of the (combined) pack and body, and so will require larger correctional forces applied to the ground (by the feet) to maintain balance and consistent velocity across the ground. This effect would be relatively minor (ie undetectable) for 1- 3 kg of water, more substantial for larger masses or smaller people (ie children).
2. The extra kinetic energy cost of the sloshing water will depend on several factors including bottle shape and orientation of the bottle in the three planes, gait style (particularly positive and negative accelerations due to heel strike and pre-toe-off ground reaction forces), etc. Typical gait entails a continuous change in horizontal velocity (of the C.O.M.) due to the (negative) heel-strike g.r.f. followed by the (positive) propulsive g.r.f. The sloshing water will create a "lag" effect on C.O.M. forward velocity, and so again may change the size of g.r.forces and thus muscle energy production; but similarly this will be relatively minor for typical water quantities. Whether that energy cost adds up to something substantial over many hours you'd have to come up with some numbers and model it.

Having said the above, I find if you have a "loose" item (like a water bottle) strapped to a pack so that it swings during walking, it is incredibly annoying - though that is probably the fear that it will come loose, or cause some damage, rather than any increased energy cost.

If sloshing water bothers you, use a soft bottle (or wine bladder).

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