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Does weight matter? - Page 2

post #31 of 38
If we're talking solely about mass and air resistance, I believe I already posted the actual formula that tells the answer above. So long as "all other things are equal" (particularly that both objects are going through the same air, and have the same drag coefficient, i.e. that they're the same general shape), terminal velocity is a constant times the square root of mass divided by frontal area. If mass divided by frontal area increases, terminal velocity increases.

To talk about skiers, the things you've got to determine by observation:

- Frontal area of a skier in a tuck does not increase linearily as mass increases.

- Ski-snow friction either has minimal effect relative to air resistance, or does not increase significantly with mass. Both of these are, I think, the case, so long as we're talking about skiers moving at significant speed (over 60 mph, definitely), on firmly packed snow wearing reasonable skis (not really short, no sandpaper on the bottom, etc.) One way to think about the first part: how fast do you think terminal velocity would be if a skier were tucking a hill in a vacuum? Personally, I think you'd need a really long hill even to get to it, and you might have a problem with the PTex melting ....
post #32 of 38
Originally Posted by montreal View Post
What I meant about mass and gravity is that gravity pulls downward with the same force for the same mass. A 1 gram feather on the moon would accelerate downward with the same speed as a 1 gram steel ball-bearing.
(edit: I totally mised the "Same mass" part. Sorry, of course it's not wrong. The rest of what I say is still true though!)

Force is equal to mass times acceleration. The acceleration due to gravity is the same for all objects, but the mass is different. Therefore, the force with which gravity pulls down is directly proportional to weight.

Your weight is the force with which gravity pulls you down towards the center of the earth.

My gut feeling would be that you'd want a good balance of weight - a balance between increased force pulling you down the hill to counteract wind resistance thanks to being heavier and lower friction with the surface of the snow thanks to being lighter.

(The friction between your skis and the surface of the snow is, all else being equal, going to be directly proportional to the force with which your skis are pushing into the snow.. ie your weight.)
post #33 of 38
Originally Posted by XJguy View Post
I dont buy it, I think that the only reason a heavier skier skis faster overall is because his mass prevents any skiing corrections from affecting his speed as much as the lighter skier. Objects in motion stay in motion unless acted upon by an outside force. Objects with more mass take more energy to slow down. Truck vs. car.

If all things are equal (mountain, gear, technique) the lighter skier would ski faster, less frontal area and less friction are the reasons.

If you got a perfectly smooth mountain and got a man that is 5'9" and weighs 175lbs and another that is 6' and weighs 220lbs, and all they had to do is tuck, no turns and no bumps on the trail, (its speed skiing in speed skiing heaven)....the smaller and lighter guy would win.

However if both men had the exact same exterior dimensions but one guy had a metal skeleton, like X-men's Wolverine making him weigh 100lbs more, he would be slower due to the additional friction, as fractional as it may be.
I'm not sure I buy this.

If two skiers are 100% identical, but one weighs 4 lbs while the other weighs 400 lbs, my money is on the 400 lb skier.

Wind resistance will quickly slow down the 4 lb skier. The 400lb skier will have much greater force pulling him down the hill, and wind resistance is probably quite a bit bigger than sliding friction on a race course.
post #34 of 38
Originally Posted by sjjohnston View Post
- Frontal area of a skier in a tuck does not increase linearily as mass increases...
Ding ding ding! We have a winner!

This is true in a tuck on skis as well as while peddaling on a bike. It's all about the MOMENTUM. All else being equal bet on bigger skiers on a windy day.

Weight makes a difference. Sometimes it helps, sometimes it hurts. But it alone doesn't win races.
post #35 of 38
Thread Starter 
Originally Posted by sjjohnston View Post
- Frontal area of a skier in a tuck does not increase linearily as mass increases.
So assuming that the drag coeficient stays the same, if you double the mass, thereby doubling the downward force, but the frontal area goes up by less than double, then the heavier skier will obtain a higher terminal velocity compared to the lighter skier, and you have the formula to prove it.

This was my original suspicion, but I wanted a second opinion.

post #36 of 38
I see the points you are making about the mass being beneficial, still I am not 100% convinced, maybe 75%. I would be interested in seeing if its so in a computer model.
post #37 of 38
Put the assumptions I made above into the computer model (that frontal area doesn't increase linearly with mass, and that snow-ski friction is either minimal or doesn't significantly increase with mass), and you'll get the same result. The math is the math.
post #38 of 38
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