or Connect
New Posts  All Forums:Forum Nav:

# My friends ski 50-60MPH+ - Page 9

The heavier skier has an advantage which has been hashed to death in other threads. It has to do with the heavier skier having more momentum to overcome friction and resistance.
Quote:
Originally Posted by Tog

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

You have to consider air drag.  It's not the gravity force component divided by mass; it's the net force, including air drag, divided by mass, and there is no mass term in the air drag. It's (m g sin(theta) - CdApV^2) /m

There is no air on the moon.   (M*g*sine(theta) - 0)/ M = m gsine(theta) -0)m  but on Earth (M*g*sin(theta) - CdApV^2)/M  is greater than (m*g*sin(theta) - CdApV^2)/m on Earth.

(EDIT) simply falling theta = 09 degrees and sine(theta) = 1, so for the video situation you could leave out the sine (theta).

EDIT 2: Re-arranging the acceleration you get  M g /M - CdApV^2/M = g- CdApV^2/M is bigger than mg/m -CdApV^2/m  = g- CdApV^2/m because

CdApV^2/M is smaller than CdApV^2/m, g - a small number versus g - a big number.

Edited by Ghost - 4/10/14 at 9:18pm
Quote:
Originally Posted by Cirquerider

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.

YOu're joking right?

Quote:

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.

So in that ^^ the friction force is proportional to the weight. That would obviously be greater for the heavier skier.

Quote:
Originally Posted by Ghost

Quote:
Originally Posted by Tog

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

You have to consider air drag.  It's not the gravity force component divided by mass; it's the net force, including air drag, divided by mass, and there is no mass term in the air drag. It's (m g sin(theta) - CdApV^2) /m

There is no air on the moon.   (M*g*sine(theta) - 0)/ M = m gsine(theta) -0)m  but on Earth (M*g*sin(theta) - CdApV^2)/M  is greater than (m*g*sin(theta) - CdApV^2)/m on Earth.

(EDIT) simply falling theta = 09 degrees and sine(theta) = 1, so for the video situation you could leave out the sine (theta).

EDIT 2: Re-arranging the acceleration you get  M g /M - CdApV^2/M = g- CdApV^2/M is bigger than mg/m -CdApV^2/m  = g- CdApV^2/m because

CdApV^2/M is smaller than CdApV^2/m, g - a small number versus g - a big number.

Ok, but what about the friction which is proportional to the weight, no?

not that simple. that friction equation is for dry friction, snow is a mix of dry and lubricated friction, lots of other variables come into play.
so the friction is actually variable depending on many things, and an higher mass will generate higher forces and higher momentum.
without doing the physics, as you accelerate you generate more heat, reduce the friction of snow, increase the momentum, which than allows you to accelerate even more and so on. all of this until wind resistance becomes to high to overcome.

http://www.swixsport.com/dav/babc49f803.pdf

Quote:
Originally Posted by Tog

So in that ^^ the friction force is proportional to the weight. That would obviously be greater for the heavier skier.

Ok, but what about the friction which is proportional to the weight, no?

The wind drag is approximately proportional to the front surface area.

e.g. a sphere would have drag approximately proportional to radius^2, and driving force proportional to radius^3.

A skiers body is a bit more complicated, but in general a bigger skier will have an advantage when straightlining. Marcel Hirscher will have an extremely hard time beating Svindal in a DH.

Regarding the OP, speed is nothing without control.

If you cannot make a turn with a radius less than the sidecut radius, or stop in a few meters you are not in control.

Quote:
Originally Posted by Ghost

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.

You are correct that gravitational force is not constant.  I see that I wrote that a ways back, but I meant to say that the acceleration due to gravity is constant.

You are correct about the effects of wind resistance as well, which I acknowledged in an earlier post.  I also noted that density becomes an issue since a skinny guy with muscle/bone density will have less volume and more weight, whereas it would be possible to have a lower density with more volume but less weight.  Weight isn't necessarily the determining factor here.

I'll admit that some of my earlier statements were wrong.  I started down the physics rabbit hole over a comment stating that gravity would cause a greater acceleration for a heavier skier.  My brain was wrapped around that thought, while others were clinging to the wind resistance concept.  It was kind of like two separate conversations going on.

Quote:

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.

If two things are equal to each other they can be used interchangeably.  So if mgSinƟ is the Force (F) driving a skier down the hill, you mean to tell me that Newton's Second Law doesn't apply and that F is not equal to ma in this case?  Because what I said was mgSinƟ=F=ma.

Regarding the ski/snow friction,  Once you get up around 60 mph, on slopes less than about 30 degrees, the wind resistance is so huge, that any difference between skis  is lost in the random static of other variables when free skiing.

Quote:
Originally Posted by Tog

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

1)  @TreeFiter:

2) @Ghost :

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

I don't think they are two different versions at all.  Its more like two different pieces to a puzzle.  My argument was focused on the effects of gravity on the skier, where Ghost was discussing the wind resistance component.  They are both right, and I discussed the effects of wind resistance to some degree in one of my posts, where I pointed out that the density, volume, and cross sectional area (all related) of the skier would be important, not just the weight.

Quote:
Originally Posted by Tog

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

The mass of the hammer contributes to overcoming the resistance of the air it is passing through on earth where the feather doesn't have the weight  and is more hindered by the air on earth.  So ya, on the moon the fat guy and skinny kid might accelerate and carry speed more equally.

Edited by crgildart - 4/11/14 at 7:21am

Pole plants and ripping 50 in the back seat.  Seems legit.

What if they tried curling with a hockey puck instead of a giant stone?  Both are slick across the ice but one will travel a LOT farther than the other from a point where they are both traveling the same speed.  That mass helps overcome friction and resistance.  The same principles apply of we tip the ice so they are traveling downhill to some degree right?  However if we tip the ice so they are traveling uphill the puck would probably travel farther right?

Quote:
Originally Posted by Jamt

Regarding the OP, speed is nothing without control.

If you cannot make a turn with a radius less than the sidecut radius, or stop in a few meters you are not in control.

QFT

Wind resistance. Anyone ride or race a road bike? You don't need an equation to understand the effort to overcome wind resistance riding 30kph vs 45.

You can apply all the formulas you want to address the question and come to a theoretical answer to the question. But in the real world when it comes to determining the combined effects of gravity, friction and wind resistance over varying pitches, snow and wind conditions, a clock will provide the only answer that matters in a race. On any given day I can beat my friend or he can beat me. He is 30 pounds lighter, has newer skis and a new speed suit. We usually wax the same or similarly. What is the real decider between victory vs. defeat?

Skill.

Any boob can point his skis straight down the hill and go scary fast. Going faster is simply a matter of having the cojones to keep pointing straight down the hill.

Quote:
Originally Posted by Tog

So in that ^^ the friction force is proportional to the weight. That would obviously be greater for the heavier skier.

Ok, but what about the friction which is proportional to the weight, no?

The coefficient of friction(mu) is so low between the skis and normal snow that the gain in weight would probably offset the gain in friction by a large margin. I bet its value is >.1. So a heavier skier does have more friction, but its still mostly a negliable force.
Quote:

So a heavier skier does have more friction, but its still mostly a negliable force.

Hardly negligible to the kid watching the heavier skier pull away and hardly negligible to the lighter, more experienced skiers that didn't make the podium again...  Why does everyone want the absolute max weight allowed in bobsled?

If you kiss the wall the heavier sled loses less speed.  If you hit a patch of slush or crud the heavier skier carries more speed through it.  Sure, a lighter skier can take a superior, smarter line to overcome the disadvantage, but all other things equal heavier is better.

According to Tipler 1975 the coefficient of friction is .05 for skis on snow. Why don't you take the free body diagram I posted above and do the calculations for a 160lb man(72.5kg) and a 200lb man(90.72kg). The value for g=9.8m/s. Pick any slope angle you wish.

Instead of making red herring arguments just do the math.
Quote:

Instead of making red herring arguments do the math.

I'd rather ski faster

As someone that does ski faster (and did so for a living at one time), in skiing, particularly in ski racing,  mass theoretically makes a difference, but in reality, skill trumps all. You don't have WC coaches encouraging their racers to get heavier. They have them working on strength, agility, balance and technique. Skills.

Quote:

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.

Line is very important, of course, but if aero is so overrated, why do national teams spend so much time in wind tunnels? Why do they use a new suit for every race? Why are there rules that prohibit utilizing non-body conforming modifications to suits that may enhance aerodynamics?

Quote:
Originally Posted by MastersRacer

As someone that does ski faster (and did so for a living at one time), in skiing, particularly in ski racing,  mass theoretically makes a difference, but in reality, skill trumps all. You don't have WC coaches encouraging their racers to get heavier. They have them working on strength, agility, balance and technique. Skills.

True, but you do hear about some of the lighter kids skiing with weight belts on..

We've had the discussion about weights on kids in other threads. I don't doubt that it might help, especially when the differences are so large relatively. I don't recommend it though. More weight with less strength and altered balance means they might glide faster, but what about when they have to turn.

Work on skills or add weight through building mass, not simply strapping it on.

Is it more important to win than to actually be better?

Quote:
Originally Posted by MastersRacer

Is it more important to win than to actually be better?

Excellent point MR!

I've never heard of kids skiing with weight belts. Bizarre. What ages and like how much weight?

^^^ Um, let's not give our app bearing speedster friends any ideas.  I'm pretty sure about the physics:

More weight = more momentum.

More momentum = more damage upon impact to themselves and others.

I've not heard of it either except that in a thread a year or two ago, a youth asked about the use of one.

Speed suits, wax and equipment choices are one thing, get the best there is. But adding weight for the sake of a theoretical advantage just seems wrong. The coach and/or parent is admitting that the kid isn't good enough or that they, the coaches, can't get their athlete to improve. It isn't the kind of lesson I'd want to be teaching. You have skills and attributes that make you who you are. If you need artificial 'enhancements' to get better then you are no different than someone taking PEDs. Be happy with being the best you can be.

Quote:
Originally Posted by tball

^^^ Um, let's not give our app bearing speedster friends any ideas.  I'm pretty sure about the physics:

More weight = more momentum.

More momentum = more damage upon impact to themselves and others.

QFT

It could go like this:

Quote:
I weigh 200 pounds, so if I add 20 pounds, I'll go faster when I straightline this trail.

Here I go....

Faster, faster, ok, time to stop.

Oops, I am not balanced properly on my skis with this extra weight. I'll lean in more. On No! I can't edge well enough to stop my self with my added momentum. Watch out! Oh crap I fell and am sliding faster with my new weight. Is that the edge of the trail? Yikes! Trees!

Next season, after I rehab this broken (insert your favorite bone here), I'll add another 20 pounds and see how much faster I can go!

New Posts  All Forums:Forum Nav:
Return Home
Back to Forum: General Skiing Discussion