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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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sk1color@do
Posts: 101 - Karma: 10
I was trying to be sarcastic, but apparently nobody got it
I still really suck at this whole forum nonsense
Mar 22 2011 2:02AM
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Static
Posts: 8581 - Karma: 4,053
this WHOLE time?!!?! well that clears up a lot.
Mar 22 2011 2:09AM
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sk1color@do
Posts: 101 - Karma: 10
god no, just with that camber shock absorber BS
I'm actually trying to find a creative way to study for my next exam, haha
Mar 22 2011 2:12AM
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Roy
Posts: 1982 - Karma: 1,240

1. 1 inch of suspension does make a big difference. F1 cars only have 1 inch suspension in the front. But maybe you're right there's not much force involved when a 1440 lbs car traviling iin excess of 215 mph at times hits a bump in the track right? Especially when you take into acount the downforce the car is producing.

2. I never disagreed with you that the ocillations of a spring are a result of the intial displacement of the spring. The displacement of the spring is caused by intial impact force, this deforms the spring which reults in its oscillations. This is where you and I deffer. See the reason why the spring oscilates is because, someone said it best, for ever action there is an equal and opposite reaction, it was deformed and therefore must dispate it's new found energy by way of vibrations to return back to it's resting state.

3. Where you are most wrong is when you disagree with me saying that the event of the impact cannot be displaced over time or distances 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"

That kind of thinking is what kept people beleiving the earth was flat all those years. What you describe is a point of contact. This where the impact event starts, though breif to naked eye, the skier will touch the snow, the force impact will increase until the skier stops moving downwards. At this point you will record maxmium downwards force from the impact. The force will then start to decrease until the skier is back in balance with the force of gravity. This is an event not a point in time. This is basic classical physics, Newton's laws in full effect. Think about it, does a car crash instantly happen? If so, how does a sensor have enough time to sense an impact force then deploy your airbag? Same with a skier hitting the ground, it takes time.

BIG POINT ON 3, the simple matter of the fact is yes you can displaces the impact forces through the time it takes for the whole impact event to happen as well as the distance the impact covers. This is what shock absorbtion is based on, it's why car have crumple zones, it's why it's better to bend your knees landing from a jump then to keep your legs straight.

Please get a grasp on classical physics if your planning on ever exploring quantum, or theoretical physics or you will be embarrased.

Mar 22 2011 3:30AM
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loganimlach
Posts: 5132 - Karma: 14,054
I beleve the term dropped earlier, "pseudoscience", describes this entire thread. I'm half drunk, and even I feel dumber from reading all of this nonsensical bullshit.
Mar 22 2011 4:19AM
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_DUSTY_
Posts: 1097 - Karma: 686
Ok Roy I'll try to clear some stuff up.

1. The suspension in an F1 is there so the wheels can absorb the impact of pebbles, its technically increasing the radius at which the body of the car travels over the pebble. The suspension on say a mountain bike is to increase the amount of time that there is a change of force on the bike and biker, this is why mountain bikes have far more suspension than an F1. Skis have relatively no impact on the absorption of the landing of a jump.

2. The reason that springs oscillate is because when a force compresses the spring it stores potential energy, the spring when it is expanding is converting the potential energy to kinetic energy, then when it starts compressing again it is converting kinetic energy to potential energy. Springs work in the same way as a pendulum that oscillates.

3. Yes the impact can happen in a greater amount time (displacement would be the wrong term, time is a scalar quantity and distance is a scalar quantity, displacement is a vector quantity,) however the final amount of force that has acted on the body has to equal the initial amount of force, thus the only way to lessen the amount of force is to convert it. It does not disappear just because your skis are longer.

You should also get a grasp on basic dynamics and kinematics, take physics 20 before trying to argue your points.
Mar 22 2011 10:33AM
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I_Am_Mod
Posts: 13214 - Karma: 3,753
one thing you should understand is that speed/velocity doesn't kill you, it's sudden acceleration/deceleration that does...

if you drop a cliff into a steep hang or hit a jump and land perfect on the landing you won't lose much of your speed because of your skis and the gradual transition from vertical to horizontal movement, if you do the same on your shoes you'll be stopped dead in your tracks and die ....
:)
Mar 22 2011 10:44AM
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Mr.Huck
Posts: 6096 - Karma: 14,903
Please forgive my ignorance.  I guess it has been a long time since my physics classes.  So basically we are assuming that air resistance plays no part here.  Do we live in a vacuum?

A wider ski will make a difference.  It will create slightly more air resistance.  This could also help to create more glide so momentum continues forward longer.  Obviously I can't say exactly how much it makes a difference but every little bit helps.  Look at wing suits.  More surface area, more air resistance, more horizontal travel. The suits worn by nordic jumpers are less dramatic, but they have had a huge impact on how far jumpers could go.  

Maybe for your thesis, you could do an experiment:

Do various vertical drops in just your ski boots and compare that to doing the same drop wearing skis.  Determine rate of acceleration and deceleration for both. 


Mar 22 2011 12:12PM
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Poikenz
Posts: 1496 - Karma: 3,888
Centrifugal force doesn't actually exist just so you know. Take a car going around a corner, you push up against the door if you are in the passenger seat. Thats from acceleration. Your body wants to keep moving forward while the car is accelerating at a different angle. Yeah I know that last part probably didn't make a whole lot of sense.

But here is how it goes while falling, lets say straight down having skis on would add to air resistance making you fall slightly slower, probably not a whole lot though.

Now when it comes to the landing having some sort of transition would help because the impact is taken over a longer period of time (look it up). Example: someone throws a baseball at you, you catch it with your arm fully extended out, when the ball hits your hand you don't move it. It's going to hurt a lot more than if you move your hand back with the ball to slow it down over a longer period of time. Thats also why foam pits don't hurt.

And have you ever seen parkour? Yeah they roll after they fall so all of their momentum or most of it is put into forward momentum instead of downward momentum which also lessens the blow, so if you are on skis they slide forward also taking away less of an impact.

WOAH! VIDEOS ABOUT PHYSICS!


Mar 22 2011 12:40PM
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ImNotARacist
Posts: 5136 - Karma: -1,030
well If I drop from somthing really high to a flat landing I lay my board down, tail to nose. Distributes the pressure from jumping throughout the flex of the board. Thats why it doesn't hurt, fuck you damn physics nerds.
Mar 22 2011 12:48PM
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sk1color@do
Posts: 101 - Karma: 10
Of course in reality air resistance plays a VERY small part, but for all extensive purposes, that resistance is negligible,
As for Roy's poor grasp on how the world works:
The moment  the Skier leaves the jump, the normal force (which is acting against gravity but is NORMAL (perpendicular)  to that of the slope of the jump) ceases to exist. Impact occurs at the time t at which this force is again present. As I said before, the amount of force exerted on the skier at that point depends on his unit vector (the derivative of his trajectory at time t) as he hits the snow. If that unit vector is  equal to the unit vector of the slope, the presence of that normal force will be very small, and only increase as the slope of the hill changes after the skier has made contact with the snow (ie the point of impact). The amount of time it takes for the slope of the hill to change does not matter as long as the skier's center of gravity does not change.
Also, that 1 inch or so of movement that F1 cars have is largely due to the tangential forces they experience during cornering and not because of bumps in the track, in fact, such courses are examined to ensure that there are no bumbs in the track specifically because the cars can not handle large wheel displacement. You are also forgetting the damper aspect of the shock absorber. This is what stops the oscillation of the spring that compensated for the initial displacement. Without this aspect car would be extremely unsafe because even minuscule displacements of the spring lead to oscillations. Without the damper to stop these oscillations, the car would be impossible to drive.
Mar 22 2011 1:13PM
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sk1color@do
Posts: 101 - Karma: 10
CORRECT!!
This actually is an example of how skis can act as a spring, unlike camber
after you land tail to bolts, your skis oscillate, just like a truck with only coil-overs and no damper, but your legs act as a damper, not a spring, creating the system necessary to survive large vertical drops
Mar 22 2011 1:18PM
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ImNotARacist
Posts: 5136 - Karma: -1,030
Change your wording a little bit. Centrifugal force definitely exists just not in the same aspect as gravity or inertia.
Mar 22 2011 1:24PM
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ImNotARacist
Posts: 5136 - Karma: -1,030
yup yup. In all honesty did this really need explaining. I though basic physics was common sense.
Mar 22 2011 1:27PM
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sk1color@do
Posts: 101 - Karma: 10
so did I....
Mar 22 2011 1:28PM
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Poikenz
Posts: 1496 - Karma: 3,888
It appears real to the object being rotated. This is because the object believes that it is in a non-accelerating situation, when in fact it is. Example: Merry go round- Yes if it is moving very fast you feel like you are going to fly off, but not because there is any force acting on you outwards, because well there isn't anything pulling you off. It is acceleration acting on the body. Sure it might "exist" because you think thats what you feel but it isn't an actual force, because the only things acting on you are gravity, the normal force, centripetal force, which is the force directed towards the center of the rotation (the pivot point) and of course velocity and acceleration.
Mar 22 2011 1:43PM
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ImNotARacist
Posts: 5136 - Karma: -1,030
Your looking at it the wrong way. You are right 100 percent but in saying that it does not exist isn't quite right. I make more sense of what I mean later. IHOP BITCH!
Mar 22 2011 2:16PM
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maxwell666
Posts: 554 - Karma: 20
lol @ reading random tidbits of peoples thoughts on the matter and trying to use big words and failing.
lots of people had the right general idea, i didnt read any of the long posts or the second page so it might have been answered but its still funny how many completely uneducated people decided to weigh in on this.
Mar 22 2011 5:10PM
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_DUSTY_
Posts: 1097 - Karma: 686
I bet I'm spewing out half bullshit and I have taken physics. However some of the stuff people are saying is just ridiculous, if you want a good answer go ask a science or physics teacher not a bunch of skiers.
Mar 22 2011 5:15PM
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Roy
Posts: 1982 - Karma: 1,240

1. IDon't make me show you calculations proving to you how much force 1 inch of suspension can absorb. Basically your saying the one inch of F1 car suspension doesn't really absorb anything but the 3 inchs of suspenion on my dirt jumper is a great absorber. Even tho I weigh about 1200 pounds lighter and travel at 200 miles less an hour. Interesting.

2. It sounds like we are in agreement on this so don't know why it has a negative tone.

Everytime something converts between potential and kinetic there is a loss of energy (if ithere isn't  tell me how so i can make a perpetual motion machine and be rich) thus resulting in a dissipation of force.

3. Force = Mass x Acceleration

We agree again. We differ because (like the other kid argueing with me) your're mistakenly assuming that all of the force of impact ocurrs at one precise moment in time which it doesn't, it takes time. This is way you can displace the impact force over time aswell as the distance.

ex. Two 200 pount guys jump off a 10 foot roof. Disregarding aerodynamics, Guy A lands on his belly, Guy B his feet. Both start they're fall with the exact same potential engery. For argument sake will say guy A doesn't get his feet or hands down. Guy A can't really do much so his impact lasts lets say .015 seconds, and the distance his impact occures over is the amount of displacement in the ground and his body. Guy B lands on his feet and bends his knees and ankles.  His impact last .25 seconds and because of bending his knees and ankles his impact last over the distance inwhich he moves those joints, lets about 2 feet..

Guy B clearly has a slower time of deceleration (or negative acceleration) and since F=MA. The Force is going to be greater in Guy A as he has the same mass but a much quicker acceleration when he hits the ground.  

INCLOSING I hope you find this helpful, I could go well over board and give you a big example with the equationsand numbers to show you why I'm right (though you did agree with some stuff but took a negative connotation) but this is NS and I feel I've already nerded it up too much. I don't know what physics 20 is (they probably don't have that here) but I can assure you I've taken more then a couple classes in physics. It's times like this that I'm glad I have a card that says that I'm right, cuz it feels like im on crazy pills right now, this stuff is so basic.

Mar 22 2011 5:23PM
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ViktorVaughn
Posts: 173 - Karma: 10
you can argue it all day by comparing arbitrary examples of 1" travel suspension but fact of the matter is the camber in your skis, even if it was 1", does not have the rigidity to make a difference. I would have thought that obvious. The reason you can ride away and not just drop those distances is...da da da da.... transfer of energy! from VVVVVV to >>>>>>>>>. Parkour guys drop crazy distances to flats but, "One more time!" they transfer energy. so once again the skis have nothing to do with anything aside from keeping you moving forward instead of having to use a dive roll
Mar 22 2011 5:31PM
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sk1color@do
Posts: 101 - Karma: 10
You're only wrong in that you assume that impact can occur over a period of time.
honestly, all I think were arguing over is the definition of impact.
If we were to get really specific, one could assume that impact never really occurs, because the center of gravity never touches the snow. What you are calling duration of impact is really duration of deceleration.
I'd say we (all 3 of us nerds) agree on 2 and (with a difference on definition) 3.
But you are very, very wrong on 1. The amount of force 1 inch of suspension can absorb depends totally on the spring constant of the coil over as well as how much pressure the damper can withstand. But this has a maximum displacement of only 1 inch (less actually, there is no way the gas in the damper could be contained at that pressure).
The comparison you use doesn't work because you are traveling at a much lower speed on your dirt bike, resulting, as you said, in a lesser force exerted when you run into bumps. Furthermore, the gas contained in a dirtbike damper does not compare to that of a F1 race car.
The downforce created by the geometry of the racecar also keeps it on the track when it encounters small displacements, such as pebbles, etc. however, if the shock absorber experiences a displacement too great (more than 1 inch) the car will crash



Mar 22 2011 6:30PM
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EmperorKuzco
Posts: 6660 - Karma: 254
is the worst class ever
Mar 22 2011 6:45PM
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skiP.E.I.
Posts: 7887 - Karma: 22,457
your equipment will dampen some of the impact and the fact that on skis you can keep moving forward, transitioning the force of the fall is an important factor. This happens even when landing flat.
Mar 22 2011 7:31PM
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ViktorVaughn
Posts: 173 - Karma: 10
skis may dampen shock through vibration but the camber is doing nothing
Mar 22 2011 8:13PM
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TPalms
Posts: 859 - Karma: 252
+k +k +k +k +k that video is my new favorite thing. Thank you.
Mar 22 2011 9:39PM
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extravert
Posts: 1113 - Karma: 3,198
1. More surface area slows you down once you land
2. Skis can slide, so when you hit all of your acceleration does not go to 0, resulting in a much softer landing
Mar 22 2011 9:40PM
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monkatizer
Posts: 4704 - Karma: 1,518
this
the question has been answered a million times over
/end thread
Mar 22 2011 9:42PM
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TPalms
Posts: 859 - Karma: 252
Disagreed. O Chem is.
Mar 22 2011 9:43PM
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monkatizer
Posts: 4704 - Karma: 1,518
. physics is the shit! sooo many real world uses!
Mar 22 2011 9:59PM
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EmperorKuzco
Posts: 6660 - Karma: 254
Agreed Chem might be worse, I guess it all depends on the teacher, I had a bitch teacher for physics so I guess that kinda swayed my opinion and there's a reason that teacher got fired
Mar 22 2011 10:19PM
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_DUSTY_
Posts: 1097 - Karma: 686
1. I am not saying one inch of suspension does not absorb very much, however it is not designed to prevent damage to the car if you drop it. Lets say you drop an F1 from a foot in the air, I will assure that the car will break. The suspension on a F1 is to roll over small cracks, bumps and rocks, not to absorb jumps. Ok technically the camber of the skis will absorb some of the impact, however the amount it absorbs is rather small.

2. Yes there is a loss of energy, however last time I looked my skis were not vibrating madly when I landed, the amount of energy lost due to the skis vibrating is very small and is not a major factor in the lessening of the impact on a skier landing a jump.

3. Like it has been stated you are technically not lengthening the time of impact, you are increasing the time of acceleration (or deceleration in this case.) a=v/t

If you want to learn why skiers can land on sloped landings read http://scienceblogs.com/dotphysics/2010/02/ski_jump_landing_and_accelerat.php

I am not talking about landing on flat ground, however I am pretty sure the amount of force of landing flat on skis and not moving anywhere and the amount of force landing flat on shoes would be comparable with a lower percent deviation. Sorry about sounding a bit like an ass, I don't mean to.

Now I am going to forget all about this stuff and go ski, its a hell of a lot funner.
Mar 23 2011 1:16AM
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Mr.Huck
Posts: 6096 - Karma: 14,903
Sorry, but statements like this piss me off. 

New Schoolers is a site for skiers, not wannabe physicists.  As skiers we are better than physicists.  Many of them never take the time to retest laws of physics.  We test them every day we click in.  We know about compression and what terrain makes it happen, even if we don't know the technical definitions.  It's like every day we hit the slopes we are collecting saying "fuck you" to Sir Isaac Newton. 

This is not an anti-education rant, but if we didn't challenge conventional wisdom on many topics, we would still be chasing animals around with spears.  
Mar 23 2011 1:52PM
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Hughes
Posts: 1023 - Karma: 941
the slope makes you slow down slower so it is not such a sudden stop
Mar 23 2011 2:19PM
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OhGoodLawd
Posts: 73 - Karma: 10
Mac Miller
Mar 23 2011 2:19PM
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Hughes
Posts: 1023 - Karma: 941
You ruined this very intelligent and well written explanation with 'funner'
Mar 23 2011 2:27PM
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chicken
Posts: 6843 - Karma: 2,121
You actually have it completely backwards. It is the physicists who took the time to retest the laws on physics (ever heard of general relativity or quantum mechanics?). It's those guys who challenged conventional wisdom and said "fuck you" to newton.

For skiers like us, everything can be explained by newtonian physics. We don't test the laws of physics every time we click in. We test ourselves against those laws.
Mar 23 2011 4:50PM
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crab_pretzel
Posts: 3919 - Karma: 622
i didnt bother reading all the long explanations, but here's my basic "for dummies" take on it:
because force (what injures you when it reaches a certain threshold)is all about mass times acceleration, its all about acceleration (because obv your mass doesnt change). basically you just loose less speed on skis because they divert some of it down hill or whatever, so you have less acceleration (deceleration here), and less force on you, so less ouch.
I know theres way more in depth explanations for this and you can get into energy and different axis, but this is a deece at a glance explanation for this
Mar 23 2011 5:57PM
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crab_pretzel
Posts: 3919 - Karma: 622
thankyou i love this one post. some people say retarded things about why lands dont hurt and stuff but whatev, they may not know the technical reasons but they sure do know basic fiziks just from riding. like if they didnt they would be able to slide a box, jump, land, or ski at all
Mar 23 2011 6:03PM
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Static
Posts: 8581 - Karma: 4,053
i thought so too, skiers are the spear hunters
Mar 24 2011 10:24AM
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ViktorVaughn
Posts: 173 - Karma: 10
Whether we get it 100% right or not atleast we're thinking about it and trying to reason. I highly recommend trying your own brain for a bit, you may need to give it a few hrs to warm up and blow away the cobwebs... Mongoloid
Mar 24 2011 10:37AM
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Ilovewhitestuff
Posts: 726 - Karma: 25
when you fall staight down on to pavement, or something hard all of your kinetic energy is tranfered into the ground andf you are decelerating rapidly, that is whats causes youto get hurt. When you go off a jump or drop when skiing only some of you kinetic energy is lost into the ground, and you keep riding away forward, you are not decelerating as fast, your momentum is just changing directions. Also there is some absorbtion by the soft snow landing, and padding in your boots. This is the reason why it is better to tuck and roll when you jump off a building or something.
Mar 24 2011 10:50AM
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Poikenz
Posts: 1496 - Karma: 3,888
Also makes you sound like a beast when someone asks you a question about how something works.
Mar 24 2011 2:48PM
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Most-D
Posts: 729 - Karma: 1,088
But as this thread proves, you do actually have to know what you're talking about.
Mar 24 2011 4:28PM
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maxwell666
Posts: 554 - Karma: 20
lol i didnt have to use much of my brain in grade 11 physics when i did it and i still dont have to now. im not saying anyone is particularly stupid, just uneducated in the topic at hand but still willing to argue it. i have no problem with the fact that some people arent as smart as me and i have no problem admitting that there are just as many people who are way smarter than me, but i do have a problem with people who are so mule headed that they have to argue about things which they have no knowledge about.

no one is perfect but everyone should try to be. in my elementary school it said above the door to "shoot for the moon and if you miss you will land among the stars"
Mar 24 2011 5:36PM
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I_Am_Mod
Posts: 13214 - Karma: 3,753
you do realize that 11th grade physics is actually one big spoof and that it's actually all about little gnomes ?


get educated son !
Mar 24 2011 6:29PM
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Mr.Huck
Posts: 6096 - Karma: 14,903
That's funny!  Over the door in my elementary school it said "Shoot spit balls at dorky kids, make them see stars and moon their parents". 
Mar 24 2011 8:00PM
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john.stossel
Posts: 943 - Karma: 5
im not trying to be mean, but how canyou go through life with the abilty to speak and function,,, and ask a question like this? i think i figured the answer to your question when i was 2, and threw a tennis ball and watched the path it followed... i think you are trolling to be honest... or 6 years old
Mar 26 2011 3:04PM
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Drail
Posts: 14041 - Karma: 10,293
in highschool I asked my physics teather how far one would have to fall to reach terminal volocity while in baggy clothing and skis on. starting the fall off a 30 degree slope with a speed of around 40km/h.

he just ignored me.
Mar 26 2011 3:13PM
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wtmtss
Posts: 110 - Karma: 12
How steep the slope is affects it a lot. Your wight is distributed over the length and width of your ski's. That way when you land, instead of being pinpointed all in your feet there's a lot more area for the shock to absorb through if that makes sense.
Mar 26 2011 3:56PM
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