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January, 2025

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Physics?

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Most-D
Posts: 729 - Karma: 1,088
I know it is pretty basic, but can somebody explain to me why it is that if i jump from a certain height, I would massively injure myself without skis, but from the same height, I can ride away clean with skis? I'd really like a good explanation for it if anyone has one that I can understand. Pink name and all.
Mar 21 2011 8:13PM
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StompinStraights
Posts: 136 - Karma: 32
Your weight is distributed over a larger area? Thats all i got.
Mar 21 2011 8:14PM
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monkatizer
Posts: 4704 - Karma: 1,518
do u mean from a jump or like jumping off somthing.

it may be because u hev more surface area with skis on and some of the down ward energy is transfered into horozontial movement because u would slide with skis on but not just boots
Mar 21 2011 8:15PM
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Mr.Turtle
Posts: 3690 - Karma: 96
something to do with the fact that your landing on an angle and moving forward it spreads out the impact, which is why landing in the flats or knuckling hurts, theres no incline to land on

just made this up
Mar 21 2011 8:15PM
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wol
Posts: 338 - Karma: 11
ur landing on a down slope. and also you have a lot more support from your boots and also bindings help from rolling ankles. hope it helps
Mar 21 2011 8:16PM
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dodgey
Posts: 590 - Karma: 266
its cuz your transferring your vertical (kinetic) energy into horizontal motion
Mar 21 2011 8:16PM
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wreck.yoself
Posts: 656 - Karma: 46
your velocity and acceleration are going straight down when you jump from high up. this transfers the kinetic energy upwards with extreme force which can damage your body. when you ski into a landing the kinetic energy is disipated through foreward motion and the slope of the landing.
Mar 21 2011 8:20PM
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BenWhit
Posts: 4654 - Karma: 3,257
the force isn't applied directly downward and distributed direct through your body. also, you're travelling with an horizontal velocity, counteracting some of the force from the fall because your not coming to a complete stop. i think. let me draw something up in MS paint
Mar 21 2011 8:22PM
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BenWhit
Posts: 4654 - Karma: 3,257
i could be entirely wrong, so take my answer with loads of grains of salt.

Click for larger image
Mar 21 2011 8:34PM
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sk1color@do
Posts: 101 - Karma: 10
Questions like this make me wonder about the State's education system
You need to remember that the hills are already at an angle, so you aren't ever really going up, mostly out. The jump follows the line your body would follow if there were to be snow there, that is, if the jump was just a roller, the jump and knuckle follow that same trajectory. So really, you aren't "falling" at all. This is why when you overshoot it hurts so much. Knuckling is a different story.
Mar 21 2011 8:40PM
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BenWhit
Posts: 4654 - Karma: 3,257
but you are falling, because you have to account for gravity. granted gravity is going to act differently because of the angles involved. you still have a y-velocity.
Mar 21 2011 8:42PM
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Most-D
Posts: 729 - Karma: 1,088
I was wondering how long it would take for a douchebag comment.
I was actually thinking more about urban.
Mar 21 2011 8:42PM
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sk1color@do
Posts: 101 - Karma: 10
It took me till college to figure it out...Says even more about the school system, huh?
Urban kinda follows the same idea, plus loose snow
Mar 21 2011 8:48PM
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sk1color@do
Posts: 101 - Karma: 10
Of course you still have a y velocity, just like you do when you're skiing (almost) anywhere else.
The diagram above describes it more or less perfectly
Mar 21 2011 8:55PM
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McFellon
Posts: 1841 - Karma: 2,005
i'm going to ask my physics teacher about it tomorrow. it helps to have a practical application (skiing) to use as a model when learning about physics. plus i'm just curious
Mar 21 2011 8:59PM
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BenWhit
Posts: 4654 - Karma: 3,257
the angles are just what goofs everyone up.
Mar 21 2011 9:01PM
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loganimlach
Posts: 5132 - Karma: 14,054
this is essentially correct, except "angular acceleration due to gravity" is a completely made up term, hahaha.

the nuts and bolts are there though.
Mar 21 2011 9:05PM
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sk1color@do
Posts: 101 - Karma: 10
of course I'm a total idiot when it comes to programming, but this video shows how much loose snow can slow acceleration.
http://www.youtube.com/watch?v=tkZ7Tyf7_YM
Clicky/Embed?
Mar 21 2011 9:05PM
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RubberSoul
Posts: 11249 - Karma: 14,642
a douchebag comment, followed by maybe the worst answer so far. i applaud you
Mar 21 2011 9:06PM
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BenWhit
Posts: 4654 - Karma: 3,257
i don't really know what i was trying to say by angular acceleration, so i threw words together. i could draw it out so much easier
Mar 21 2011 9:07PM
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lukemeahan.
Posts: 1689 - Karma: 330
haha this a great thread.
Mar 21 2011 9:08PM
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sk1color@do
Posts: 101 - Karma: 10
In what way was anything I said wrong?
Mar 21 2011 9:09PM
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manabadman
Posts: 616 - Karma: 256
If you are on the roof of a building (or any raised object), you have gravitational potential energy. When you jump off the roof this g.p.e is converted into kinetic energy and you start to fall. If you were not wearing skis and were to land on flat concrete, your kinetic energy would be transferred to the floor in a very short space of time (a fraction of a second). This strong decelarative force is transferred to your bones and they would probably break. If you were to jump with skis onto some snow with a bit of a transition the deceleration happens over a longer space of time and the overall decelerative force is not as large, so you get to ride away clean. The skis and boots would also take some of the force.
Skis dont always help though e.g. Tanner at Chad's.
An excellent thread however, I feel this is the direction we need to be taking newschoolers.
Mar 21 2011 9:16PM
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sk1color@do
Posts: 101 - Karma: 10
Fuck Karma
This isn't quite right
Though skis help the rider match the angle of the knuckle/slope, they don't have much to do with the landing. Think about sno-skates, they have a very small surface area, but they can still hit the huge jumps other skiers do.
Mar 21 2011 9:25PM
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Mr.Huck
Posts: 6096 - Karma: 14,903
This thread reminds me of something.  Years ago there was a plane crash in Eagle County, Co.  It was a small private plane and everyone on board was killed.  That was sad.  A flight instructor at Eagle County airport was telling some people about it and one person asked, "what killed them".  The instructor thought about it for a second and replied "sudden stoppage". 

Others have stated correctly in this thread that rapid deceleration is what causes you to compress, collapse and get injured.  If you are dropping off a ten foot wall in your shoes and feel that the impact is significantly greater than if you did the same drop on skis, it probably is.  The reason is one of two things.  Either the skis displace the impact over a wider area, as others have said, or the width of the ski allows you to fall at a slower rate.  My theory is that it is a combination of both of those factors. 

So either the skis allow you to drop slower or they slow the rate of deceleration.  As a guy who was skiing and jumping before wider skis, I can tell you that the impact of landing on wider skis seems to be less than on skinnier skis.  Hope that makes sense.  
Mar 21 2011 10:32PM
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Jennings
Posts: 2190 - Karma: 90
haha no.
Mar 21 2011 10:44PM
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sk1color@do
Posts: 101 - Karma: 10
+k to someone that understands physics
Mar 21 2011 11:13PM
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BenWhit
Posts: 4654 - Karma: 3,257
WHAT ABOUT ME ASSHOLE
Mar 21 2011 11:15PM
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sk1color@do
Posts: 101 - Karma: 10
fixed
++k, you also gave the best answer
Mar 21 2011 11:18PM
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BenWhit
Posts: 4654 - Karma: 3,257
because it allows the skier to maintain a similar x-velocity as when they left the jump, like you said. take a long jumeper, they undergo a serious vertical drop, but that horizontal velocity allows them to ride with the steep landing at high speed. i don't know how to explain it. look at an air plane landing, because they land at an angle with a ridiculously high horizontal velocity, there isn't a bonecrushing impact. physics sucks
Mar 21 2011 11:25PM
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RudyGarmisch
Posts: 25443 - Karma: 13,844
Your downward motion is converted to lateral motion by the transition of the jump.
Mar 21 2011 11:27PM
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BenWhit
Posts: 4654 - Karma: 3,257
prime example of what we are discussing

Mar 21 2011 11:35PM
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sk1color@do
Posts: 101 - Karma: 10
1 is more or less right, the vertical momentum isn't "converted" but absorbed, as you state in 2
The reasoning in 3 is also correct, that the vertical component is the same regard less of the horizontal, but I would say that this component is absorbed through deformations in the snow/skis/legs. Another thing to think about is that if there is no skis/deck/blades, the horizontal component would also become part of the impact forces. Using skis allows those forces to remain in the horizontal direction instead of becoming moments (basically a force about a point, creating rotation)
Mar 21 2011 11:36PM
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BenWhit
Posts: 4654 - Karma: 3,257
this would be centripetal (or centrifugal, i forget) acceleration as you transition from the downward landing to the flat bottom. centrifugal force causes your body to compress as you round the transition and your direction changes axes.
Mar 21 2011 11:40PM
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sk1color@do
Posts: 101 - Karma: 10
Truth
Mar 21 2011 11:48PM
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Roy
Posts: 1982 - Karma: 1,240

So if you jump from the same height on skis or just in your shoes regardless of the landing (flat or sloped) the impact to your body will be greater in shoes because greater ability of skis to dissapate the energy of the impact. It works in two ways.

1. The ski. Most skis are cambered, this acts a shock obsorber (think leaf springs on a truck) this helps disappate energy. Energy also gets disappated through the tip and tail of the skis by way of vibrations which are induced from the force of the impact. With shoes all you have is thin rubber sole to obsorb the impact.

2. Because skis have a realitvly low co-efficent of friction, they will slide and thus incase the duration of the impact event. Where as with shoes you can only hope to increase this duration by rolling out. Say you jump off a 10 foot high roof, on skis you land and slide say 8-10 feet, in shoes you can only roll 4-5. In both cases the kinetic force of the fall will be the same, but with the skis on the force has twice the distances to dissapate in.

Mar 22 2011 12:03AM
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Loco-Deer-Slayer
Posts: 1401 - Karma: 701
If I ever make a troll thread... I'm asking a question about physics.
Mar 22 2011 12:14AM
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sk1color@do
Posts: 101 - Karma: 10
The absorption of impact has very little to do with the cambering of skis, and the resulting vibrations are only due to the elasticity of the skis (see BenWhit's post about elastic/inelastic collisions above)
Duration of the impact also has nothing to do with it. The total impact is lessened simply because the horizontal component is not turned into a moment about the skiers feet. When someone jumps wearing shoes and does not roll, the horizontal force turns into a moment about the feet, driving the head into the ground. 
Mar 22 2011 12:19AM
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_DUSTY_
Posts: 1097 - Karma: 686
I have not read all the replies so this may have been said. If you are landing flat on skis the only way it hurts less is because of the skis which spread out some of the force and they absorb some of the impact, however the amount that skis help landing completely flat is not much. If you land flat on skis and move forward then the impact will also be less since you will be converting more of the vertical force acting on your body to horizontal force however your body will still have to absorb a great deal of the force exerted on it.

However if you are landing on the landing of a jump the reason that it does not hurt is different. When you leave the lip of the jump you have two major forces acting on you, the force of gravity, or the vertical force, and your momentum, also known as your horizontal force. The amount of force you feel on your body is the amount of force the grounds exerts on you.

For example say you jump off a house that is twenty feet high the entire time you are accelerating towards ground. Lets say you accumulate 800 N (Newtons) of force, when you hit the ground the grounds exerts 800N of force on you. (One of newton's law.)

Now lets say you jump off a cliff of equal size when you land on a slope the ground will exert a Normal Force (as in a 90 degrees angle from the point you land on.) Normal Forces have two components, a vertical and horizontal component since they are at an angle. When you land on an incline a lot of the vertical force exerted on your body is converted to horizontal force since the ground is acting on you at an angle.

If you want to know some of the math read this guys article or you can pm me.

Also I am a bit shaky on dynamics right now, the particle physics are fucking with my head.
Mar 22 2011 12:31AM
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_DUSTY_
Posts: 1097 - Karma: 686
I have not read all the replies so this may have been said. If you are landing flat on skis the only way it hurts less is because of the skis which spread out some of the force and they absorb some of the impact, however the amount that skis help landing completely flat is not much. If you land flat on skis and move forward then the impact will also be less since you will be converting more of the vertical force acting on your body to horizontal force however your body will still have to absorb a great deal of the force exerted on it.

However if you are landing on the landing of a jump the reason that it does not hurt is different. When you leave the lip of the jump you have two major forces acting on you, the force of gravity, or the vertical force, and your momentum, also known as your horizontal force. The amount of force you feel on your body is the amount of force the grounds exerts on you.

For example say you jump off a house that is twenty feet high the entire time you are accelerating towards ground. Lets say you accumulate 800 N (Newtons) of force, when you hit the ground the grounds exerts 800N of force on you. (One of newton's law.)

Now lets say you jump off a cliff of equal size when you land on a slope the ground will exert a Normal Force (as in a 90 degrees angle from the point you land on.) Normal Forces have two components, a vertical and horizontal component since they are at an angle. When you land on an incline a lot of the vertical force exerted on your body is converted to horizontal force since the ground is acting on you at an angle.

If you want to know some of the math read this guys article or you can pm me:



Also I am a bit shaky on dynamics right now, the particle physics are fucking with my head.

Mar 22 2011 12:31AM
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sk1color@do
Posts: 101 - Karma: 10
Everything you said is right, except that the vertical force is not converted to horizontal until the slope you land on changes from very vertical (the backside of the knuckle) to less vertical (the average slope of the mountain)
Just play Line Rider, it explains everything
Mar 22 2011 12:38AM
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Roy
Posts: 1982 - Karma: 1,240

I very much disagree with you.

1. The camber of the skis will have a noticable effect. Say you have 1 inch of camber, thats like having a shock absorber with once ince of travel, it makes a difference. Good example try poping with cambered skis then try with flat or reversse camber skis. A real world application of this is leaf-spring suspension systems, which you can easily see on any pick-up truck. So if your saying thats wrong, you also saying a suspension system used in vehicles (while not the best) is completly ineffective.

2.  Yes skis are elastic which is why they vibrate, but what are vibrations, well they are a form of energy. So when the impact occures if the impact energy is transformed into vibrations there is a greater dissapation of froce. Even the stiffness of ski matters, a stiffer ski will have a frequency and produce shorter wave length, where as a not so stiff ski will have a lower frequncy and produce a longer wave length. The reason this matters is because transfering the energy of the impact is all about making the impact last as long as possible over the greatest distance possible. So a loose ski vibrating with a longer wave length disperce the impact over more time and a longer distance then a stiff ski would.

3. I want to call you an idiot...so I am, you're an idiot. Duration of the impact is a huge part of how a collision is dissapated, that and the distance the impact occures over. These two factors have a direct result in how much force is transfered from one object to another. I think that teach that in grade 10 physics. An example of this is a down sloped landing. Why does a steep angled landing allow you while your sking to dissapate force? Because of the way it transfers your downward motion into forward mortion. But how does it do that? It does it because the time inwhich the impact takes place is longer as well as it takes place over a longer distance. Thus indication that the duration of the impact has quite a bit to do with it.

Mar 22 2011 1:21AM
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DIX~
Posts: 4695 - Karma: 6,502
My fucking god.  The amount of stupidity in this thread is just incredible.
Mar 22 2011 1:24AM
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p-fo
Posts: 9372 - Karma: 6,156
downward momentum gets transferred into forward momentum more easily without the friction of your heels digging in. plus your skis spread out the impact some, too.
Mar 22 2011 1:42AM
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sk1color@do
Posts: 101 - Karma: 10
Sorry everyone else
Sure, 1 inch of travel in a spring will dampen the initial impact of the landing, but only 1 inch.
In comparison, a truck or other similar spring device used for shock absorption is designed to allow for much bigger displacement, as well as much smaller point forces acting upon those springs. While these are effective for trucks and the like, they are not effective for the skiing situation because 1 inch of travel is simply not enough to compensate for the amount of force the skier will exert in the vertical direction.
Vibrations are also accounted for in this spring/damper example used in trucks and cars. You are correct that these systems use a camper-like system to account for sudden displacement, but the resulting oscillations of the spring are ONLY a result of the initial displacement. These systems use a damper to lessen these oscillations so that the truck will only bounce as a result of the initial displacement and not subsequent oscillations of the spring part of the spring/damper system.
Really, the whole idea of "dispersing the impact" is incorrect, because there is point at which the skier is not in contact with the snow and a point when he/she is in contact. I define this moment as "the moment of impact."
Thus impact occurs at time t and at no other time before or after
What everything comes down to is if I hit a jump and my unit vector at the moment I hit the snow and the surface of the knuckle are TANGENT, then I will feel as though there was essentially NO impact whatsoever. If these 2 vectors (the surface of the knuckle can be represented as a unit vector as well) are not equal, then I will feel an impact which magnitude is related to the angle between those 2 vectors and the force the skier has in the direction of his or her unit vector. A greater angle results in a more severe force exerted upon the skier by the knuckle.
Mar 22 2011 1:45AM
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Static
Posts: 8581 - Karma: 4,054
ahh yes the fat ski parachute hypothesis, lessened by rocker
Mar 22 2011 1:50AM
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sk1color@do
Posts: 101 - Karma: 10
This, together with the shock absorber camber allows skiers to fall from tremendous heights without so much as some vibration
Mar 22 2011 1:52AM
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sk1color@do
Posts: 101 - Karma: 10
Not trajectory, but my unit vector (which describes only my direction of travel)
And by knuckle I mean the backside of the knuckle
my park terminology =/= good
Mar 22 2011 1:56AM
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Static
Posts: 8581 - Karma: 4,054
i was actually being sarcastic, I seriously doubt a fatter ski provides enough wind resistance to make a difference, maybe when flying over a football field in a ski jumping competition......
Mar 22 2011 1:56AM
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sk1color@do
Posts: 101 - Karma: 10
check your meter
Mar 22 2011 1:58AM
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