I’ve been watching skiing at the Olympics, and I notice that the commentators always seem to refer to “getting air” - that is, flying through the air - with disapprobation. Why?
Seems to me that there’s less friction in air than in snow, so getting air would mean a faster time. But my wife pointed out that a skier in the air is moving in a parabola rather than a straight line, which is not as fast.
I don’t ski myself, so any skiers out there who can enlighten me?
When your skis are in contact with the slope (snow), gravity is pulling you against the slope, and the slope converts some of that into a horizontal (forward) pull. This is what makes the skier accelerate forward, or maintain forward motion.
When you’re in the air, you’re only accelerating downward. Your forward speed can only decrease because nothing is pushing/pulling you forward. Granted, some of the downward speed gets converted to forward speed when you hit the ground, but that’s a very inefficient process. Most of that impact is absorbed by your legs, and some of it goes towards compressing the snow.
As a lesser concern, you cannot maneuver while you’re in the air. Ideally, you’d stay in contact with the ground and have the maximum amount of time to set up for the next turn.
Skiers have to steer. Skis steer by pressing on snow. Flying through the air, skiers cannot turn or control their direction.
Skiers maintain control by the contact between their skis and the snow. When that contact is lost, they lose control and wipe out. You can see it on the wipeouts.
Landing is an opportunity to lose balance. Losing balance is loss of control. At high speeds, this often means wipeout. At minimum, even a minor loss of balance is a loss of time. In a sport where the difference in gold and bronze is 0.9 secs, you don’t want that.
When skis are in contact with snow, they are pushing the skier. If the skier wants to carve tight corners and shorten the overall distance of travel, he needs his skies pushing on snow. If he gets air, his ski’s are moving in the previous direction, not making the turns sharp. Ergo, he travels farther (independent of any vertical distance change). That means more time.
Also while in the air, a skier can’t tuck into an aerodynamic shape and needs to flail to maintain balance. Their wind cross section is thus significantly larger.
Wow, that is such a good answer and one I wouldn’t have considered. Sometime the SDMB is amazing. Wish I’d had you for high-school physics.
Used to race slalom as a young lad. The coaches were very concerned about air, and we were taught to unweight or even jump slightly before any lip or bump that could cause air. Done right it was very effective at eliminating or reducing air time. It was made clear to us that time in air would reduce our speed.
Control is certainly an equally important issue and while in air you are not in control and landing is an opportunity to wipe out. However, even in straight sections where steering was not at issue, air was avoided. A skier getting less air time was always at an advantage to one who managed it poorly.
My physics is rusty, but that does not sound right to me. The slope is not actively pushing you forward. Either way, the only force acting on you, or, at any rate, the only source of potential energy to accelerate you, is gravity, pulling straight down. If the ‘slope’ was vertical you would be in ‘air’ all the time, and reach the bottom a lot quicker. And on a regular slope you do not stop moving forward when you are in the air, because you have forward momentum.
I guess you are right about energy being absorbed by legs and snow when you land, though.
I’m pretty good at physics, and I don’t buy scr4’s explanation. I do, however, but the one about inevitably coming out of the aerodynamic tuck when a skier is in the air.
No, scr4 is correct.
You have a vector of acceleration. Gravity pulls straight down. Ergo, when you are in the air, you are not accelerating forward (you actually slow down because of drag). You accelerate straight down. When you hit, almost all of that potential energy goes into your legs flexing.
When you are on a sloped plane, gravity pulls straight down still, obviously. However, your normal force (the upward force exerted by the ground) is not vertical, it’s perpendicular to the plane. Thus the acceleration is downhill at some magnitude less than gravity.
So when your skis are touching the ground, you are being accelerated downhill/forward. When you are in the air, you are not.
http://library.thinkquest.org/16600/intermediate/force.shtml
Anyway, for a quick thought experiment drop a ball onto a ramp at the same time you just let go of a ball on another ramp. The one that drops will take longer to get to the bottom, because the one touching the ramp accelerates forward instantly, where as the dropped ball has to contact the ramp to gain any forward velocity.
Since we’re getting physical here, I’ve been thinking during the Olympics about figure-skating in microgravity. Any thoughts?
Just remember that it is a sport of what seems to be minor things meaning huge consequences:
Winners separated by HUNDRETHS of a second. To the naked eye, sometimes the top 5 (or even top 6 or 7) would cross the finish line at exact moments.
So, even if catching an iota more air than someone else only costs 1/100th of 1 second, that could take you from “national hero with a lifetime of endorsements and income” to “some guy that ran the downhill in 2010”.
.
Something doesn’t seem quite right here. It’s true that while airborne, the only acceleration is vertically downward, but since the course is sloped, motion in a vertically downward direction has a forward component relative to the slope.
To illustrate, consider two skiers travelling at the same speed who have travelled the same distance down the course. One is on the ground, the other is airborne directly above him, relative to the slope. Note that this means that the airborne skier will be ahead of the other skier, relative to the horizontal. So the airborne skier will drop and contact the ground at about the point where the other skier will have reached.
It seems to me that any differences will be due to friction and air resistance. Also, flying into the air and landing is probably not good for one’s form, as noted.
So how about a physical expiriment?
Take a 45 degree inclined plane of ice. Start two icecubes at the top at the same time. Cube A makes a straight path line to the end of the plane. Cube B has a small ‘jump’ about 1/4 of the way down the track in which the ice cube goes airborne staying just above the plane a lands back on the plane 3/4 of the way down. Which cube wins? Cube A.
No. Assume a vaccuum. At the instant you start the experiment with one skier directly above the other (note that initial speed doesn’t matter), the skier on the ground starts accelerating forward. During the time the other skier is accelerating straight down, and not forward. By the time the skier in the air hits the ground, he will be behind the other skier, because the other skier got a “head start” on acceleration.
The airborne skier’s connection with the ground is inelastic - he doesn’t bounce, and only a tiny amount, if any, of that momentum gained from falling will be translated into forward momentum. In effect, the skier on the ground gets a head start that is equal to the time it will take the airborne skier to fall from whatever height he starts from.
If you add in air resistance, the skier in the air is slowing down, while the other is not (but he will accelerate slightly slower, as will the airborne skier), unless he has somehow exceeded his terminal velocity on the slope. This is highly unlikley.
You are thinking of it like the dropped bullet/fired bullet thought experiment. The reason that is not applicable here is because the skiers have different forces acting on them - one has a force straight down, one has a force parallel to the slop of the hill, instead of just the downward force of gravity.
I can buy loss of speed via greater air resistance due to less “tucked” shape, and a loss of some speed via impact shock when landing, but the notion of gravity accelerating you more quickly due to contact with the snow vs falling through air is a bit hard to understand.
Has it not been adequately explained in my posts and the provided link?
No, the skier in the air is ahead of the other skier, at the start. He is directly above him relative to the slope, which means that in vertical terms he is ahead. If they are stationary, he will drop vertically to the point where the other skier will be at the same time. While accelerating vertically, the skier in the air does accelerate forward, relative to the slope.
You can think of the downward force acting on either skier as consisting of two components, one perpendicular to the slope, one parallel to it. Being in contact with the ground merely counterbalances the perpendicular component. It doesn’t somehow magnify the parallel component. In fact, it reduces it a little, due to friction. The airborne skier, meanwhile, still has the perpendicular component and the parallel component. Both skiers are of course also subject to friction from the air. Maybe the airborne skier has a little more air resistance, what with his ski surfaces being exposed to the air, and perhaps greater turbulence around him. Don’t know, I’m not an aerodynamicist.
(I can see this thread turning into another “plane on a treadmill” :))
I’m spot on with you. I’m not seeing the whole “contact with the ground makes you faster via gravity” argument.
Contact with the ground gives more control - sure
Contact with the ground means a better aerodynamic tuck - quite possible
Contact with the ground means less momentum loss due to impact shocks when landing after being airborne - sure
Contact with the ground accelerates you faster because of gravity “pulling” you down the slope - not so much
The other skier doesn’t get a “head start”. You’re giving him a straight up, no-quotes head start. You’re also neglecting the pre-flight situation. The airborne skier doesn’t get as much horizontal push- true. But he does get some at the bump that sent him into the air. The downhill guy went down and forward at the same time. The air guy went forward fast, but not down, while entering flight. He then went down fast, but not forward, while leaving flight. It balances.
You’re technically right, ivn, but all of that isn’t an explanation for the time difference. It’s simply a longer path with less balance. Going too far up is bad for the same reason going too far sideways is bad. It’s longer.