or Connect
New Posts  All Forums:Forum Nav:

# My friends ski 50-60MPH+ - Page 8

LOL, Newton with apple and grape from tower too...
Free fall in no atmosphere is different.  Two Words... Pinewood Derby...  Weight trumps aerodynamics on a slope,  This is why bobsled races have very strict weight limit requirements and validation.

Quote:
Originally Posted by crgildart

LOL, Newton with apple and grape from tower too...
Free fall in no atmosphere is different.  Two Words... Pinewood Derby...  Weight trumps aerodynamics on a slope,  This is why bobsled races have very strict weight limit requirements and validation.

Weight has nothing to do with it. Its almost entirely about wind resistance.

Pinewood derby, if I remember correctly, has a specific weight requirement, so that all cars are more or less the same weight.  The key to a fast pinewood derby car is eliminating drag, both in the wheels and the aerodynamics of the car.  It is advantageous to build the car at the maximum allowable weight because the track is only inclined for about the first quarter of the track.  After that it is flat, and it becomes a question of inertia (See Newton's First Law).  Given two cars of equal mass, it once again comes down to eliminating drag.

As for bobsleds, I don't know much about those.  But I can assure you that if you drop a bobsled and a feather on the moon, they will still hit the ground at the same time because that's how gravity works.

I actually come down on the side of the weight people here. I think weight gives more momentum which helps in overcoming perturbations in one's course such as small piles of snow. Presumably that's the weight in the pinewood derby overcoming friction. It's endlessly debated. Is there an actual answer?

Remember when the Austrians complained that Lindsey Vonn was too heavy? Daron Rhalves was very light for a speed skier and did well though.

Let me add something to all the tall tales here. I skied a day of powder in the trees last week and recorded it on my phone with motionx. Max speed was 36, probably done returning to base.
Quote:
Originally Posted by TreeFiter

Weight has nothing to do with it. Its almost entirely about wind resistance.

Pinewood derby, if I remember correctly, has a specific weight requirement, so that all cars are more or less the same weight.  The key to a fast pinewood derby car is eliminating drag, both in the wheels and the aerodynamics of the car.  It is advantageous to build the car at the maximum allowable weight because the track is only inclined for about the first quarter of the track.  After that it is flat, and it becomes a question of inertia (See Newton's First Law).  Given two cars of equal mass, it once again comes down to eliminating drag.

Many years pinewood derby experience here.  Weight limit is 5 ounces.  A 5.0 ounce brick with no shaping usually beats a 4.5 ounce perfect airfoil shape when the wheels and axles are done the same.  That's just how it is.  Kids in jr racing will add weight belts when they can too.

Quote:
Originally Posted by TreeFiter

Weight has nothing to do with it. Its almost entirely about wind resistance.

Pinewood derby, if I remember correctly, has a specific weight requirement, so that all cars are more or less the same weight.  The key to a fast pinewood derby car is eliminating drag, both in the wheels and the aerodynamics of the car.  It is advantageous to build the car at the maximum allowable weight because the track is only inclined for about the first quarter of the track.  After that it is flat, and it becomes a question of inertia (See Newton's First Law).  Given two cars of equal mass, it once again comes down to eliminating drag.

As for bobsleds, I don't know much about those.  But I can assure you that if you drop a bobsled and a feather on the moon, they will still hit the ground at the same time because that's how gravity works.
Eh, your 100% wrong. The coefficient of drag ranges from 0-1, putting on speed suit will only change it by maby .1-2. That's a marginal increase compared to weight. It all comes down to f=ma. This should be readily apparent on the ski slope. At 200lbs on a snowboard I would be faster than most normal sized skiers on flats.

If aero was so important with ski racing they woouldnt be skiing with uncovered boots. In bike racing wearing shoe covers will shave a couple seconds off a 50k time trial. 3k aero rims will only shave a couple seconds too, and an aero helmet..a couple seconds. That's on a 50k bike ride, not a short 1-2 mile ski run. Aero is wayyyy overrated.
Edited by clink83 - 4/10/14 at 3:30pm

It's called the square-cube law:

Take two objects (or people) with the larger exactly twice in all dimensions as the smaller:

The larger will have eight times the weight and therefore eight times the gravitational force acting upon him.

The larger will have four times the frontal area and thus four times the drag, assuming equal drag coefficients (which is a valid assumption except for relatively minor Reynolds number effects).

The effect of ski friction against the snow is less clear. Classical friction theory, assuming equal friction coefficients, says that the larger would have eight times the friction, however assumption of equal coefficients without knowing the frictional characteristics as a function of area and pressure is a rather large assumption.

Therefore, at higher speeds, where aerodynamic drag predominates, the larger will be faster.

Quote:

Eh, your 100% wrong. The coefficient of drag ranges from 0-1, putting on speed suit will only change it by maby .1-2. That's a marginal increase compared to weight. It all comes down to f=ma. This should be readily apparent on the ski slope. At 200lbs on a snowboard I would be faster than most normal sized skiers on flats.

If aero was so important with ski racing they woouldnt be skiing with uncovered boots. In bike racing wearing shoe covers will shave a couple seconds off a 50k time trial. 3k aero rims will only shave a couple seconds too, and an aero helmet..a couple seconds. That's on a 50k bike ride, not a short 1-2 mile ski run. Aero is wayyyy overrated.

There is only one force driving a skier down the hill when skiing.  That force is gravity.  Because gravity is essentially acting straight down, and a skier is on a slope (or inclined plane) the vector component acting on the skier is equal to mg*Sinθ, m being mass, g being acceleration due to gravity, and θ being the slope angle.  The rate at which this object will accelerate down that slope (the primary factor in determining how fast that skier will go) will be based on F=ma.  F is the vector component I just mentioned (mg*Sinθ).  m is the mass of the skier, and is the acceleration that the skier will experience.  We have already seen that F=mg*Sinθ so we can substitute mg*Sinθ in and we have mg*Sinθ=ma.  Since we have m on both sides, we divide each side by m canceling out the effects of mass.  Considering that Weight = mass * acceleration due to gravity, if mass is not a factor, neither is weight.  Like I said "Weight has nothing to do with it."

If we were talking about carrying speed over flat ground while decelerating from resistive force, weight would make a difference, but in terms of two objects sliding down a hill, it makes no difference.

When we consider wind resistance, once again its a question of F=ma.  F in this case is the resisting force created by the air/wind.  More weight=more mass, so if we increase m, it will require a greater force to decrease the acceleration of a skier.  Assuming two skiers of the same shape and size, but different weights, the wind resistance will be equal.  So F is the same for both.  We can see that ma=F=2m*0.5a  So if you double the mass, the opposing acceleration is cut in half.  If you were to increase the mass by 10%, the opposing acceleration would be reduced by one tenth.

Now this is where it gets interesting.  Typically a person's volume and therefore cross sectional area increases with their mass.  Assuming two skiers of equal density, with different masses, the differences pretty much cancel each other out.  If the densities of the two skiers are significantly different, there will be a difference in cross sectional area that can either negate or intensify the effects of wind resistance discussed above.  In other words, high density would be low surface area with lots of mass (a lead broomstick), and low density would have high surface area with little mass (a spread out bed sheet).  So yes weight can make some difference in terms of a skiers reaction to wind resistance.

This discussion really started over the comment :

Quote:

They would have the same speed. 30mph is 30mph. They would accelate differently though. The heavier skier would accelerate faster than the light skier because weight is a bigger factor than the coefficent of drag.
The difference between speed and acceleration is a concept lost on many though.

And as I said, the acceleration of a skier has nothing to do with the weight.

Tog brought up momentum, and in a way he is correct.  As I demonstrated above more weight is less impacted by wind resistance, which is essentially the result of inertia, which is closely tied to momentum.

Quote:
Originally Posted by crgildart

Many years pinewood derby experience here.  Weight limit is 5 ounces.  A 5.0 ounce brick with no shaping usually beats a 4.5 ounce perfect airfoil shape when the wheels and axles are done the same.  That's just how it is.  Kids in jr racing will add weight belts when they can too.

As I said before, pinewood derby tracks are not a continuous slope.  Once they hit the flat, the weight becomes an issue.  If all things were equal, with the exception of weight, the cars would all reach the end of the slope at the same time, but the heavier cars would pull ahead once on the flat. In the skiing situation that started this whole physics discussion it was suggested that a heavier skier would accelerate downhill faster than a lighter skier.  Its just not true.

Quote:
Originally Posted by Bill Miles

It's called the square-cube law:

Take two objects (or people) with the larger exactly twice in all dimensions as the smaller:

The larger will have eight times the weight and therefore eight times the gravitational force acting upon him.

The larger will have four times the frontal area and thus four times the drag, assuming equal drag coefficients (which is a valid assumption except for relatively minor Reynolds number effects).

The effect of ski friction against the snow is less clear. Classical friction theory, assuming equal friction coefficients, says that the larger would have eight times the friction, however assumption of equal coefficients without knowing the frictional characteristics as a function of area and pressure is a rather large assumption.

Therefore, at higher speeds, where aerodynamic drag predominates, the larger will be faster.

Gravitational force is constant at the surface of the earth, as is the acceleration it creates.

And there is also a direct and positive correlation between attainable speed and intelligence (IQ).

I've worked it out here:

This is why the skier will always be faster than the snowboarder.

TreeFiter is correct. Gravity is not a "force", it is an acceleration and, to any reasonable approximation, is constant everywhere on earth (9.8m/s^2). WTF, doesn't anyone except TreeFiter go to school in America?

Quote:
Originally Posted by TreeFiter

As I said before, pinewood derby tracks are not a continuous slope.  Once they hit the flat, the weight becomes an issue.  If all things were equal, with the exception of weight, the cars would all reach the end of the slope at the same time, but the heavier cars would pull ahead once on the flat. In the skiing situation that started this whole physics discussion it was suggested that a heavier skier would accelerate downhill faster than a lighter skier.  Its just not true.

More mass, same resistance = more velocity

The acceleration is constant, the force is not.

F= (M1 x M2 x G)/R2

Where M1 is mass of earth (contstant) and M2 is mass of object (not constant, varies with object), G is the universal gravitational constant and R is the Radius between C.G's of the earth and the object (constant at surface of earth).

The point I was trying to make, perhaps not too well, is that the larger, heavier object experiences more gravitational force, but does not have the same proportion of increased aerodynamic drag.

Quote:
Originally Posted by TreeFiter

Gravitational force is constant at the surface of the earth, as is the acceleration it creates.

...... force can be constant by approximation, but the magnitude of the force differs between objects with different mass and thus the terminal velocity differs....as Bill shows.

Force drag = Force gravity

Very simple here guys...

Quote:
Originally Posted by crgildart

More mass, same resistance = more velocity

More mass, same resistence = more retained velocity as an object travels along a flat plane.

Quote:
Originally Posted by Spooky

...... force can be constant by approximation, but the magnitude of the force differs between objects with different mass and thus the terminal velocity differs....as Bill shows.

Force drag = Force gravity

Very simple here guys...

If Force drag=Force gravity, there is no net force, and no acceleration.  When was the last time you pointed your skis down the fall line and stood still?

Quote:
Originally Posted by TreeFiter

Quote:
Originally Posted by Spooky

...... force can be constant by approximation, but the magnitude of the force differs between objects with different mass and thus the terminal velocity differs....as Bill shows.

Force drag = Force gravity

Very simple here guys...

If Force drag=Force gravity, there is no net force, and no acceleration.  When was the last time you pointed your skis down the fall line and stood still?

Northstar, towards the bottom, (well it's a long way), in heavy powder. I hear Wold Creek is like that too.

High wind going uphill, not steep slope.

Warm wax, very cold snow...

.......ok, just being a smart a**

.....edit: Trying to ski switch with skins on

Edited by Tog - 4/10/14 at 8:55pm
Quote:
Originally Posted by TreeFiter

If Force drag=Force gravity, there is no net force, and no acceleration.  When was the last time you pointed your skis down the fall line and stood still?

Less flapping and more reading........Terminal velocity.

Quote:
Originally Posted by TreeFiter

More mass, same resistence = more retained velocity as an object travels along a flat plane.

And a higher terminal velocity on a slope.

Quote:
Originally Posted by Bill Miles

The acceleration is constant, the force is not.

F= (M1 x M2 x G)/R2

Where M1 is mass of earth (contstant) and M2 is mass of object (not constant, varies with object), G is the universal gravitational constant and R is the Radius between C.G's of the earth and the object (constant at surface of earth).

The point I was trying to make, perhaps not too well, is that the larger, heavier object experiences more gravitational force, but does not have the same proportion of increased aerodynamic drag.

Increased gravitational force is irrelevant because the rate of acceleration is based on force and mass.  Higher weight means higher mass, which means more force is needed to get the acceleration.  In other words all objects are accelerated equally by gravity on the surface of the earth.

Quote:
Originally Posted by crgildart

And a higher terminal velocity on a slope.

How the heck did we get to terminal velocity from someone claiming that a heavier skier would be accelerated down the hill more by gravity?

Gravitational force is not constant; as stated above it varies with the mass of the two objects attracting each other.  Gravitational force attracting skiers to Earth is called weight, and 225 lb skier has more weight than 150 lb skier.

The acceleration of the skier will equal his weight times the NET force acting on him, which is  the component of his weight ( mg sine(theta) ) - the drag force (Cd Ap V^2).  Because the drag force is relatively lower compared to the gravity force for a heavy skier,  the acceleration is (MgSin(theta) - Cd Ap V^2)/M greater for the heavier skier than (MgSin(theta) - Cd Ap V^2)/M for the lighter skier.  Also terminal speed is higher for the heavier skier, when MgSin(theta) = Cd Ap V^2.

Quote:
Originally Posted by pat

Oh I think my feelings will be ok. At least we can all agree on the bacon thing.

pat you are so wrong on this.....

I am fairly certain I can hit 50-60 mph on most of the groomers here... I am in normal clothes on 91mm under foot ski unpossible right?

from the bend at 1:08 to cattrack at 1:53 is a straightline distance of 1 kilometer. It takes me 45 seconds to go 1 kilometer which is 80 km/h or 49 mph. considering this speed is straight line and i was actually turning, i thiink its safe to say 50 mph is more than possible. I could go faster but according to the internet I am a no talent hack.

Quote:
Originally Posted by Josh Matta

pat you are so wrong on this.....

I am fairly certain I can hit 50-60 mph on most of the groomers here... I am in normal clothes on 91mm under foot ski unpossible right?

from the bend at 1:08 to cattrack at 1:53 is a straightline distance of 1 kilometer. It takes me 45 seconds to go 1 kilometer which is 80 km/h or 49 mph. considering this speed is straight line and i was actually turning, i thiink its safe to say 50 mph is more than possible. I could go faster but according to the internet I am a no talent hack.

Josh, You have all kinks of ski talent, but sometimes not much tack. I like your posts , you say a lot,but you can back it up, that what ticks some people off. Keep skiing and posting

So we apparently have two versions of physics. We need @Jamt to referee here.

1)  @TreeFiter:

Quote:

There is only one force driving a skier down the hill when skiing.  That force is gravity.  Because gravity is essentially acting straight down, and a skier is on a slope (or inclined plane) the vector component acting on the skier is equal to mg*Sinθ, m being mass, g being acceleration due to gravity, and θ being the slope angle.  The rate at which this object will accelerate down that slope (the primary factor in determining how fast that skier will go) will be based on F=ma.  F is the vector component I just mentioned (mg*Sinθ).  m is the mass of the skier, and is the acceleration that the skier will experience.

We have already seen that F=mg*Sinθ so we can substitute mg*Sinθ in and we have mg*Sinθ=ma.  Since we have m on both sides, we divide each side by m canceling out the effects of mass.  Considering that Weight = mass * acceleration due to gravity, if mass is not a factor, neither is weight.  Like I said "Weight has nothing to do with it."

If we were talking about carrying speed over flat ground while decelerating from resistive force, weight would make a difference, but in terms of two objects sliding down a hill, it makes no difference.

2) @Ghost :

Quote:

Gravitational force is not constant; as stated above it varies with the mass of the two objects attracting each other.  Gravitational force attracting skiers to Earth is called weight, and 225 lb skier has more weight than 150 lb skier.

The acceleration of the skier will equal his weight times the NET force acting on him, which is  the component of his weight ( mg sine(theta) ) - the drag force (Cd Ap V^2).  Because the drag force is relatively lower compared to the gravity force for a heavy skier,  the acceleration is (MgSin(theta) - Cd Ap V^2)/M greater for the heavier skier than (MgSin(theta) - Cd Ap V^2)/M for the lighter skier.  Also terminal speed is higher for the heavier skier, when MgSin(theta) = Cd Ap V^2.

So....what's the difference here with David Scott's demo on the moon? Instead of straight down, the objects are sliding on a slope. If Scott had dropped them on a frictionless sloped table, the results would be the same, no? Yet, apparently on earth, not counting friction, heavier objects go downhill faster than lighter ones? Don't get it.

Blue above. The hammer still has more weight than the feather. Moon has gravity or the Astronauts would be floating. It's 1/6th of Earth's.

What's the difference here besides air resistance and a slope?:

http://youtu.be/KDp1tiUsZw8

Quote:

Eh, your 100% wrong. The coefficient of drag ranges from 0-1, putting on speed suit will only change it by maby .1-2. That's a marginal increase compared to weight. It all comes down to f=ma. This should be readily apparent on the ski slope. At 200lbs on a snowboard I would be faster than most normal sized skiers on flats.

If aero was so important with ski racing they woouldnt be skiing with uncovered boots. In bike racing wearing shoe covers will shave a couple seconds off a 50k time trial. 3k aero rims will only shave a couple seconds too, and an aero helmet..a couple seconds. That's on a 50k bike ride, not a short 1-2 mile ski run. Aero is wayyyy overrated.

Aero is important to ski racing, there are rules that prohibit certain modifications for the purpose of aerodynamic advantage. For instance, you can't add piping to speed suits to break airflow. Aero is not overrated in ski racing. Just take my word for it. There is a reason the national teams use wind tunnels for training.

For the sake of the op's query about his friends going so fast it isn't critical, but two guys side by side straight lining, the better aero will go faster sooner.
Treefitter is using the equations wrong. You can't substitute an equation into itself. If you draw the freebody diagram properly it makes more sense.

That's not including drag of course. There is a reason the #1 law of physics is draw the diagram first, then apply the proper equation.
Edited by clink83 - 4/10/14 at 9:13pm

Thanks to Treefiter for the basic science and laws of physics that govern all acceleration, kinetic energy and ultimately speed on an inclined plane.

Ironic who the physical law "deniers" are here.

Quote:
Originally Posted by MastersRacer

Aero is important to ski racing, there are rules that prohibit certain modifications for the purpose of aerodynamic advantage. For instance, you can't add piping to speed suits to break airflow. Aero is not overrated in ski racing. Just take my word for it. There is a reason the national teams use wind tunnels for training. I don't need to take anyone's word for it, because the math is pretty simple.

For the sake of the op's query about his friends going so fast it isn't critical, but two guys side by side straight lining, the better aero will go faster sooner.
Aero is very much overrated. Youre only making tiny changes to the CD by tweaking aero stuff, which has a tiny effect on drag. Frontal area(ie maintaining a tuck) has a much bigger effect on drag. The magnitude of difference in weight is going to greatly outweight the small change in CD a skin suit provides.
If you would like me to prove this mathematically I can, but I don't want to turn my computer on to type it out.
What seperates the winners from losers is largley line selection. Its that simple.
Edited by clink83 - 4/10/14 at 9:20pm
New Posts  All Forums:Forum Nav:
Return Home
Back to Forum: General Skiing Discussion