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How fast do you think you ski on piste?

 Poster: A snowHead
Poster: A snowHead
foxtrotzulu wrote:
@dsoutar,
Quote:

Of course the heavier skier may have a larger frontal area and hence generate more air resistance...

Are you saying that @SnowDave has moobs?


It's all muscle!
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 Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
The point about weight us that cross-sectional area increases less rapidly than mass so heavier skis will have a bigger weight:cross sectional area (i.e. drag) than lighter skiers.
It's also (sort of)why elephants have to have big ears to cool down
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 Well, the person's real but it's just a made up name, see?
Well, the person's real but it's just a made up name, see?
Penry wrote:
@uktrailmonster, The bit that says that gravitational force = mass x gravity. There will be a component parallel to the slope.


You are getting confused between force and acceleration. Acceleration due to gravity is a constant for all masses. If you drop a tennis ball and a cricket ball from the same height, they will both hit the ground at the same time and with the same speed. The cricket ball will hit the ground with a higher force, but not any faster. Their relative masses are irrelevant. It's the same with a pair of skiers of different masses as far as their acceleration and therefore speed due to gravity is concerned.

The variables that really matter are aerodynamic drag (based on the drag coefficient and frontal area), frictional force between skis and ground and last, but not least, stability. A bigger, heavier skier is actually likely to create more friction and more aerodynamic drag, both of which would imply a slower speed. But on the other hand, more mass will generally help with stability and help prevent them actually taking off with aerodynamic lift, which is a potential problem with speed skiing. Also probably why adding weights to a bobsleigh is beneficial, not because it increases speed due to gravity (i.e. bad physics, lol), but because it makes the vehicle inherently more stable at high speed. Same with speed skis. Heavier skis are generally more stable at speed. The extra mass itself does not make gravity accelerate the body any faster or slower. It's a constant.
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@uktrailmonster, I think Jedster is more nuanced, wind resistance is a function of frontal area, whereas mass (of a human) is proportional to volume.

So force increases as ^3 , resistance as ^2 so benefitting the heavier skier.
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 Anyway, snowHeads is much more fun if you do.
Anyway, snowHeads is much more fun if you do.
http://www.math.utah.edu/~eyre/rsbfaq/physics.html
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@jedster,
Quote:

The point about weight us that cross-sectional area increases less rapidly than mass so heavier skis will have a bigger weight:cross sectional area (i.e. drag) than lighter skiers.
It's also (sort of)why elephants have to have big ears to cool down
Are you saying that @snowdave has got big ears as well as moobs?
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Then you can post your own questions or snow reports...
@foxtrotzulu, I don’t want to turn this into a helmet thread, but I reckon under a lid, all ears are created equal!
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 After all it is free Go on u know u want to!
After all it is free Go on u know u want to!
It is quite astonishing how much faster things get with a catsuit vs ski kit...
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under a new name wrote:
It is quite astonishing how much faster things get with a catsuit vs ski kit...


I bet the ears and tail slow you down though

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i reckon i must be the slowest skier in the world, if i"m in a group i"m always at the back, which of course means less rest, but i"m a one week a year holiday skier and i dont care.....so there
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@davidof, not the way I wear them Twisted Evil
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 And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
@uktrailmonster, @under a new name, the way I thought about it is as follows...

In the diagram below, the system is in equilibrium if the resistance Fr is equal to the sum of the forces W & Fn in the opposite direction to Fr. W is dependent on mass, so Fn must also be dependent on mass. Therefore the force opposing friction increases with mass.

My thinking could be way out here but, if the coefficient of resistance is very low, an increase in mass will mean a small increase in friction but a large increase in its opposing force and the object will accelerate down the hill. Intuitively, adding more weight to the blue box will move it. I'm not sure that what I've said stands up to much scientific scrutiny though.


https://en.m.wikipedia.org/wiki/Normal_force#/media/File%3AFriction_relative_to_normal_force_(diagram).png

Apologies, I can't make the link work
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I would like to add to the above physics arguments that I speculate skis will reduce friction by melting the snow and self lubricating due to pressure effect. This will be more the notable in a carve.

Does anyone know if this is the case?
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 You know it makes sense.
You know it makes sense.
@motdoc, someone did the physics of this theory a while back and it seems pretty conclusive that the pressure increase isn’t anywhere near enough to cause the melting effect.
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 Otherwise you'll just go on seeing the one name:
Otherwise you'll just go on seeing the one name:
Did some googling and this would seem to suggest that melting is probably part of the equation.

My phone said 95 kph thats my story and Im sticking to it
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 Poster: A snowHead
Poster: A snowHead
I just bought a shooting laser rangefinder that has a speed function on it. I might test it out as a means of checking some of the phone app data that other people are using.
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 Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
@Penry, that's for dry friction between two flat planes sliding against each other.

Skis are a way load more complicated with fluid resistance, melting, etc. all getting into the mix. @motdoc, melting defo part of it
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Well, the person's real but it's just a made up name, see?
@under a new name, Yes, I've come to the conclusion my explanation is bollux too.

The thing is, my experiences say the heavier skier is faster. My son is 5 11 to my 6 3 but 55kg to my 90. So surface area similar, massive weight difference and I fly past him when we both point our skis down the hill. And I think he had a benefit when it came to friction between ski and snow; a few things indicated his skis were slipperier than mine.

And when I was a kid the same applied to soapbox derby cars. Basically as a cub scout whittling a small brick of wood into the shape of a car, attaching wheels then racing each other down a multi-laned ramp. The winning tactic was not to streamline the block at all but to hollow it out and stuff it with heavy things, e.g. coins, batteries.

The link that @davidof posted is interesting but I'm struggling to work out what it's saying.
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@Penry,

it's what I said in the earlier post wind resistance increases more slowly than mass - so bigger skiers gain more from gravity than they give up from drag. It is that simple
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i'm not trying to say that weight of the skiier/snowboarder doesn't matter, but as someone that has tried to catch a run away snowboard i can categorically state that its not the be all and end all! shitoffashovel springs to mind!
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@eddiethebus, that is what I touched on earllier with the runaway old style brakeless skis. No payload pushing them down into the snow, no extra wind resistance and gravity is constant.
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jedster wrote:
@Penry,

it's what I said in the earlier post wind resistance increases more slowly than mass - so bigger skiers gain more from gravity than they give up from drag. It is that simple


But what are bigger skiers gaining from gravity alone? They don't accelerate any faster or slower due to gravity and would all free fall at the same 9.81 m/s^2 or less depending on their aero drag.

The big guys definitely lose out on aero drag, given their larger frontal area and assuming the same Cd (i.e. similar clothing and stance).
The big guys also lose out on friction, whether significant or not.

Clearly these inherent losses can be mitigated in racing by the sheer strength of bigger guys (even simply shoving themselves out of the gates) and the inherent stability you get from increased mass.
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@uktrailmonster, friction is complicated. Beyond my pay grade. Wind resistance is not, mass increases faster than drag does, so F(g) increases faster with mass than F(d) so more mass = more acceleration, in the absence of other factors.
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under a new name wrote:
@uktrailmonster, friction is complicated. Beyond my pay grade. Wind resistance is not, mass increases faster than drag does, so F(g) increases faster with mass than F(d) so more mass = more acceleration, in the absence of other factors.


More mass does not equal more acceleration due to gravity. The force (due to gravity) increases with mass, but acceleration (due to gravity) is a constant. People are intuitively suggesting that greater weight/force = more acceleration. Well it doesn't in my book (or anyone else who believes Newton) unless you apply an external force other than gravity e.g. a jet pack. In simple terms heavier things require larger forces to accelerate than lighter things (F=ma), but when discussing gravity the F and m effectively balance each other out to give a constant acceleration i.e. "g" (Fg = mg where g is a constant)

When we start talking about opposing forces like friction and wind resistance, they are all generally increasing for a larger mass, while gravitational acceleration (and its component acting along the slope) is constant for all masses.
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@uktrailmonster, yes and no. More mass = more FORCE due to gravity. Two falling bodies of different masses absent any other influences accelerate at the same rate. I posted the equations above. The different masses cancel.

However, we are discussing whether a heavier person accelerates against e.g. wind resistance faster or slower than a lighter person and the loosely theoretical and empirical evidence says, yes they do.

Or, rather, empirical evidence seems to suggest they do and loose theory suggests plausible explanations.
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under a new name wrote:
@uktrailmonster, yes and no. More mass = more FORCE due to gravity. Two falling bodies of different masses absent any other influences accelerate at the same rate. I posted the equations above. The different masses cancel.

However, we are discussing whether a heavier person accelerates against e.g. wind resistance faster or slower than a lighter person and the loosely theoretical and empirical evidence says, yes they do.

Or, rather, empirical evidence seems to suggest they do and loose theory suggests plausible explanations.


Okay I kind of see where you are coming from now i.e. the drag force on a lighter body has more effect than drag on a heavier body. That does make sense actually.
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 And love to help out and answer questions and of course, read each other's snow reports.
And love to help out and answer questions and of course, read each other's snow reports.
Experiencially, big f*kers always overtake little f*kers on the cat track. Ergo fatboys go faster than runts. Sod the physics.

wink Smile NehNeh
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Arctic Roll wrote:
Experiencially, big f*kers always overtake little f*kers on the cat track. Ergo fatboys go faster than runts. Sod the physics.

wink Smile NehNeh


That must explain why I'm so slow on cat tracks Embarassed
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 You know it makes sense.
You know it makes sense.
In the context of skiing there are significant variiables such as how aerodynamic your clothes are and how well waxed your skis are. These will both effect how well you go down a cat track
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@Rabbie, but can you beat a lone ski on the loose?
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 Poster: A snowHead
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jedster wrote:
@Penry,

it's what I said in the earlier post wind resistance increases more slowly than mass - so bigger skiers gain more from gravity than they give up from drag. It is that simple


it really is

> Sod the physics.

No the physics says that too. Gravity is a constant so what you need to look at is aero drag.

Aero drag is a function of cross-sectional area and speed, not mass. Ski straight down a slope twice, once with a 25 kg weight strapped to your back and once without. Your terminal velocity will be higher with the weight because the gravitational pull will be higher but the aero drag will be the same. Of course a heavier skier is bigger but is a skier 10% larger in each dimension will be 33% heavier (1.1^3 - 1) but will only have 21% more area (1.1^2 - 1). (as was said above).
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 Obviously A snowHead isn't a real person
Obviously A snowHead isn't a real person
Okay I think we are all in agreement about the effect of wind resistance favouring the heavier guys. If you reverse it with a strong tailwind, the lighter guys will get blown faster down the slope and in a headwind the lighter guys will be slowed more.

What I've noticed on shallow cat tracks (where the component of gravity acting down the slope is pretty small) is that ski bases/wax are the main factor in how fast you can actually glide. On slightly steeper slopes, it's more down to aerodynamics e.g. tuck vs standing up. If there is a strong headwind, that's when the fatties seem to have the biggest advantage - which makes sense re. the above discussion.
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Well, the person's real but it's just a made up name, see?
Nothing passes me in a straight line.
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davidof wrote:
jedster wrote:
@Penry,

it's what I said in the earlier post wind resistance increases more slowly than mass - so bigger skiers gain more from gravity than they give up from drag. It is that simple


it really is

> Sod the physics.

No the physics says that too. Gravity is a constant so what you need to look at is aero drag.

Aero drag is a function of cross-sectional area and speed, not mass. Ski straight down a slope twice, once with a 25 kg weight strapped to your back and once without. Your terminal velocity will be higher with the weight because the gravitational pull will be higher but the aero drag will be the same. Of course a heavier skier is bigger but is a skier 10% larger in each dimension will be 33% heavier (1.1^3 - 1) but will only have 21% more area (1.1^2 - 1). (as was said above).



http://youtube.com/v/5C5_dOEyAfk
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Posted this on the "straight line" thread as well but caught this on my trip last week to La Plagne. This is on the blue "Tunnel" run.

An example of someone not able to control speed, not paying attention, not caring or a dangerous combination of all 3.

I slowed down intending to stop but it appeared that both were uninjured and I knew his group were behind me so I carried on, not wanting to "inflame" the situation I did however let him know what I thought of the idiot on the way past. (Had he been prone on the floor or apparently injured then yes, of course I would have stopped).



http://youtube.com/v/y8iukJojlAc


Last edited by Anyway, snowHeads is much more fun if you do. on Fri 9-02-18 18:31; edited 3 times in total
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@Fridge03, how come you didn't stop? You do know you are duty bound by the FIS rules to assist?

http://www.fis-ski.com/mm/Document/documentlibrary/Administrative/02/04/30/10FISRulesofConduct-English-A4_Neutral.pdf
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That's quality Asshattery - was the asshat the guy in the white helmet? If not I guess he is sprinting up the slope to punch the tool out - in the interests of entertainment at least you should have stuck around.
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kitenski wrote:
@Fridge03, how come you didn't stop? You do know you are duty bound by the FIS rules to assist?

http://www.fis-ski.com/mm/Document/documentlibrary/Administrative/02/04/30/10FISRulesofConduct-English-A4_Neutral.pdf


I have edited my post explaining (it's also on the video description anyway).

The "victims" large group were directly behind me, I was with a group that had gone on ahead. I slowed down, saw that both parties were standing up and seemed uninjured. Stopping could have A, inflamed the situation as I had some choice words to say and B, I don't think my assistance was required. Had either one of them been prone on the floor, not moving or in distress then of course I would have stopped.

But thanks for questioning my actions rather than the oxygen thief who caused the incident in the first place.
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Dave of the Marmottes wrote:
That's quality Asshattery - was the asshat the guy in the white helmet? If not I guess he is sprinting up the slope to punch the tool out - in the interests of entertainment at least you should have stuck around.


Yeah, the guy in the white helmet was the asshat! Great term, will use that more. Haha.

As I said, I toyed with stopping but it appeared no one was injured, he has his group with him and mine were a good distance ahead by that point.
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It seems pretty obvious to me that heavier skiers go faster. As mass increases there's more force pushing you down the hill. As size increases there's more air resistance to slow you down, but mass increases more quickly than frontal area so heavier wins. In a no-skill, straight lining through a speed trap scenario.

A few years ago I was in a tour group on a day out in Lech from St Anton. There was a wee speed trap at the bottom of a gentle blue. We all went through it on the way to the lift and initially I was pleased to find I was the fastest (64kmh). I was a bit less pleased when I looked at the rest of the group and realised that the speeds were pretty much proportional to weight. I reckon that speed trap could easily have been calibrated to estimate weight.

Last week SkiTracks told me my max speed was 115kmh. This was clearly a blip and I don't believe it, but I do believe that I was regularly doing 80-90kmh early in the mornings when the pistes were still empty. I've done over 90kmh on a bicycle and ski slopes are generally much steeper than Scottish roads, so it doesn't seem impossible to me for skiers to hit these kinds of speeds.

https://physics.stackexchange.com/questions/59815/is-a-heavier-skier-faster has some reasonable looking calculations, although I think @under a new name was better at physics at school than I was.
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