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Mrs MTG is helping junior with her homework.
One of the things she mentioned she has learned from it herself is that as warm water is less dense, it creates less drag, so it is easier to swim through.
However, I would have thought that if the water is less dense, then to displace the same mass of water, the swimmer would float lower, creating more drag.
Do the two cancel each other out, or is there really an advantage to swimming in warm water?
What about salt water, which is denser at the same temperature? Does that make it harder to swim through than fresh water?
If it's less dense you'd float less. Which means more of your effort would go into staying afloat than going forward.
It's much easier to swim in salt water for this reason afaik, and also much easier to swim with a (tri/swimming) wetsuit on.
However (and this may be out of scope for this question) your body has to work harder to keep warm in cold water, but likewise in really warm water you'd struggle to keep your body temp down so your performance would suffer.
So I'd guess you'd be fastest in cool-ish salt water with a wetsuit on 🙂 Competition pools btw feel fairly cool, and colder than the leisure ones - so I guess there's an optimum balance there for speed purposes.
colder than the leisure ones - so I guess there's an optimum balance there for speed purposes.
Surely the few degrees difference we're talking about here would not result in a change in the density of the water sufficiently enough to have any meaningful affect on performance ?
Warm air is less dense than cold air so presumably easier to run through, throw things at, etc, but I have never heard of air temperatures being given when world records in track/athletic events are announced.
I really can't believe that they drop the temperature in competition pools by a few degrees to reduce the water density 🙂
It's much easier to swim in salt water for this reason afaik, and also much easier to swim with a (tri/swimming) wetsuit on.
Not sure about that (as someone who has spent much of their last 25 years doing watersports in warm, cold, fresh and salt, shorts, wetsuit and drysuit).
The colder the water, the easier it is to swim as there are fewer old dears breast stroking side by side having a matter poluting the water with a gallon of perfume in the fast lane.
Nothing to do with physics.
IME energetic swimming in a warm pool is very unpleasant, you feel like you're slow cooking.
Cold water is very difficult to swim through. At try it at -2c, see how far you get...
I've been trying to think of the question in relation to ships.
With all the ships travelling all over the world, I would guess that it is probably common knowledge amongst those involved in the business that ships use less fuel, or travel faster, in either warm or cold water.
As a ship sails from say, Scandinavia to Mexico, the water gets less dense.
That means it floats lower in the water, displacing a greater volume, but the same mass of water.
So, is it the mass or the volume that creates the drag?
Then, what about the propeller? If the water is less dense, it won't work so efficiently. Does the same go for a swimmer's arm strokes and kicks?
poluting the water with a gallon of perfume
Quite.
Surely the few degrees difference we're talking about here would not result in a change in the density of the water sufficiently enough to have any meaningful affect on performance ?
I don't know. I was thinking of the Plimsoll Line. The change in density with temperature obviously has an effect on ships.
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Not sure about that (as someone who has spent much of their last 25 years doing watersports in warm, cold, fresh and salt, shorts, wetsuit and drysuit).
I have swum a couple of triathlons without a wetsuit, and my times were considerably slower than with one on. Almost anyone will swim faster in a swimming wetsuit. They make you more buoyant, so there's less drag, and generally have a pretty slick surface, further reducing drag. Why do you think swimmers set all those records in the suits at Beijing that were subsequently banned? Those were essentially thin wetsuits.
Done triathlons in pools, lakes & various sea's in all conditions. The best conditions for fast times were deep cold calm water with wetsuit on.
The main reason people can swim faster in wetsuits is the suit irons out all the stroke deficiency that the swimmer has.
What you're saying pleaderwilliams, contradicts what she's been told at school, but agrees with [url= http://en.wikipedia.org/wiki/Triathlon_equipment ]Wikipedia[/url]
[i]"Another advantage of a wetsuit is the added buoyancy. It provides triathletes with a considerable advantage."
"No wetsuit with a thickness of greater than 5mm may be used."[/i]
So it looks like making the combined volume of swimmer and trisuit less dense, effectively making the water more dense, the swimmer is faster.
Does the temperature, and therefore the density, of the water affect the strength of the swimmers stroke?
Can you push harder against cold water?
The density of water doesn't change by any noticeable amount with temperature:
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However, the viscosity does decrease quite significantly at higher temperatures, which would reduce drag (although it might also have an effect on how much "purchase" you could get on the water to pull yourself along).
If it's less dense you'd float less. Which means more of your effort would go into staying afloat than going forward.
Except we are all positively buoyant (some more than others 8) ), so no effort is required to stay afloat.
dp
geoffj, have you ever been swimming?
Geoffg, have you ever been swimming?
Silver personal survival (merit) badge thank you very much! What's your point caller?
I hadn't thought about viscosity, mrodgers.
I found a graph, not that I properly understand it.
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Does that really mean that water at 0C is twice as thick as water at 25C? That doesn't sound right.
From my very old mech eng knowledge, drag is composed of form drag, skin friction and wave drag. Wave drag is caused by the raising of an amount of water to make the wake, hence it'll be lower if you make little or no wake. This is the reason there's a limit on the number of strokes you can take underwater in competitive breast stroke. If you stay on the surface, wave drag will be less in less dense water.
I suspect this was just a throwaway line from the teacher - I'd love to see just how big the effect is for a swimmer.
Wetsuits help reduce skin friction and also add boyancy to the legs (which don't float in my experience).
What's your point caller?
His point is that you ate talking bobbins 🙂 We are positively buoyant overall not neutrally, we float a bit; but we tend to float in a way that is not very useful for making progress because whilst out torso may float upwards out legs do not. This coupled with the need to keep our head up means we need to expend effort to remain horizontal to cut through the water easily. Having more buoyant legs in a wetsuit means the thrust vector can be more in the direction of travel and less upwards.
Almost anyone will swim faster
in a swimming wetsuit.
Which is why it's so unfair that I'm slower 🙂
I think the most important benefit of a swimming wetsuit to your average swimmer (or STW triathlete) is that the buoyancy helps you to keep your form better, which in turn makes you more streamlined, reducing drag.
Edit: Thrust vector?! It's a bit early in the day to be over analysing to that degree Molly surely? 🙂
This coupled with the need to keep our head up means we need to expend effort to remain horizontal to cut through the water easily. Having more buoyant legs in a wetsuit means the thrust vector can be more in the direction of travel and less upwards.
That's not what you said though. [b]means more of your effort would go into staying afloat than going forward.[/b]
Looking at the viscosity graph, I guess there could be a circa 15% difference in viscosity between a coolish sea temperature and a warmish pool temperature.
I once worked on a research boat on Lake Ontario. It was a hot summer's day, and we'd finished for the day, but for some reason (and I can't remember why), we were sat on the hook out in the middle - you couldn't see land. We all decided to go for a swim. I jumped in, my brain was telling me that this was open water outside so must be the sea, but the relative density was telling me otherwise and I was sitting lower in the water than I was expecting - quite a weird sensation until your brain readjusts.
This is poor technique, you don't need to keep your head up. To swim efficiently you roll from side to side and breathe, if you poke your head up then your legs go down and you create drag.This coupled with the need to keep our head up means we need to expend effort to remain horizontal to cut through the water easily.
If open water swimming you'll need to sight every now and then so this is unavoidable to a degree but good technique will minimise the negative effects.
Warm air is less dense than cold air so presumably easier to run through, throw things at, etc, but I have never heard of air temperatures being given when world records in track/athletic events are announced.
Thats why velodromes are very warm, typically around 25-26C. It makes a definite difference to times in a velodrome.
Man from NASA says teacher is right
http://science.howstuffworks.com/swimsuit-swim-faster1.htm
On the viscosity theme: http://www.discovery.com/tv-shows/mythbusters/videos/swimming-in-syrup-minimyth.htm
I assume that air resistance in a velodrome has more affect on the performance of a competitor than air resistance has on the performance of a sprinter, shot putter, high jumper, etc.
[quote=geoffj ]Except we are all positively buoyant (some more than others ), so no effort is required to stay afloat.
So I was once told by an instructor when doing personal survival or something similar. My response to which was to sit on the bottom of the pool. Ironically, the very best (male) swimmers are also unlikely to float.
Does that really mean that water at 0C is twice as thick as water at 25C? That doesn't sound right.
Yes, according to that graph, water at 25C is twice as viscous as water at 0C. To work out what effect this has on drag, you need to work out the Reynolds number of the flow (Density*Velocity*Width of object/Viscosity). The graph below shows how the drag coefficient varies with the Reynolds number for a sphere (for a person swimming the numbers will be different, but the shape of the graph will be similar):
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For a typical swimming speed I imagine you'd be somewhere to the left of the "dip" at point 5 (I wouldn't know exactly without more information). Halving the viscosity will double the Reynolds number, but depending on which part of the graph you are in this might not have much of an effect.
Surely the few degrees difference we're talking about here would not result in a change in the density of the water sufficiently enough to have any meaningful affect on performance ?
London Velodrome is (aparently, I've not been) under negative pressure to simulate being at altitude, marginal gains and all that.
Competition pools feel bloomin cold, and there's often a temperature reading (or at least a whiteboard with the number written on it) so you can get your excuses ready.
The best conditions for fast times were deep cold calm water with wetsuit on.
What significance does water depth have on performance ?
The warmer the water the faster you go in sprints as there's less drag - up to about 32°C. However, overheating soon reduces performance so the best compromise for anything other than sprints is around 26°C. Edit: the race limit is 29°C max
A wet suit used to save me eight to ten minutes on a 3800m swim: best pool time around 1h 10, best open water time 1h 02. The main difference was only needing a lazy four beat kick with the suit which kept the heart rate down and left me fresh for the bike and run.
What significance does water depth have on performance ?
You have more water underneath you, which helps you to float. 😉
What significance does water depth have on performance ?
IIRC shallow pools "reflect" your bow wave back at you.
Joking aside, it's well known that fast pools are deep and have splash decks to break waves and limit reflections so that swimmers are in the cleanest water possible; turbulence slows you down. All the swim and tri clubs in my local town were consulted when the new pool was being designed. The result is a fantastic pool, which we can't use at present because the slabs around it reach over 60°C on sunny days.
[quote=ernie_lynch ]What significance does water depth have on performance ?
With shallow water you get increased wave formation due to the "bottom effect", which is due to reflections off the bottom. This results in increased wave drag, which is basically down to having to swim uphill to get over the wave.
most people with a heathly BMI are neutrally buoyant at the surface, it's the amount of air in the lungs that helps us to float / sink (very overweight people can be positively bouyant due to the subcutaneous fat layer that provides buoyancy)
BMI is very definitely the wrong measure to tell whether somebody will float or sink!