# How do ski jumpers NOT get fall damage?

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The distance they jump/the speed at which they fall seems like it would be impossible to land without falling flat on your face. There is obviously some cool physics going on, but please.

In: Mathematics

The short answer is that force is a vector. So explaining this more ELI5, if you jumped from a high surface on to the ground, all of the force of gravity from you falling is returned right back up in to you. This is what causes the risk of injury to your feet, ankles, legs, knees is all of that force pushing back on those.

However ski jumpers don’t land on a flat surface, they are landing on a very sloped surface. So the force is much more in a horizontal direction rather than pure vertical. It should also be noted that where they land is covered with very loosely packed snow. This snow will also absorb that force and disperse it as they are landing.

Throw a ball directly at a wall and see how far it travels afterwards, i.e. how much of its momentum is conserved

Now throw a ball at a 45deg angle and see how far it travels afterwards, i.e. how much of its momentum is conserved

Now throw a ball at the smallest angle you can and see how far it travels afterwards, i.e. how much of its momentum is conserved

This is the basic idea. Ski jumpers aren’t just landing ANYWHERE, they are landing in a landing zone that is built at an angle to make the landing easier.

Same thing with mountain bike tricks and all of that other X games stuff – when you land at an angle like that, much of your momentum is conserved as FORWARD momentum in the direction you were (and still are) travelling.

The landing being on snow helps some (relative to concrete), and the large skis also help absorb some of the impact (they “spread” out the force of landing somewhat)

It’s not the fall that kills you but the sudden stop. You’ll notice ski jumpers land on a downslope not flat ground. If they were to land on flat ground it would be like hitting a wall. Since the downslope roughly matches the trajectory of their fall and slowly flattens out, there is not sudden stop. The forces get converted from downward motion to forward speed.

There’s a lot of skill involved in making sure you don’t land without falling flat on your face.

Part of that skill involves holding your body in such a way that you maximize lift, which includes holding the skis a certain way. The skis themselves are different from normal downshill skis – considerably wider and longer than normal. This gives them more control and makes it so that they’re more gliding than falling.

Another skill is learning how to land correctly. When executed properly, the landing is spread out longer to minimize the change in velocity (from hitting the ground).

Finally, ski jumpers land on a slope, not on a flat. Again, this serves to make it so that they lose less velocity when hitting the ground. Instead of abruptly stopping (which results in a very large change in velocity over a short time, which in turn gives a high force), they can maintain both their horizontal and vertical velocity – essentially, they’re still “falling” down the slope. And being in contact with the ground, they can then shed their velocity in a more controlled and drawn-out fashion, which minimizes the force they have to deal with.

To use an analogy: Falling a from a building is like driving your car straight into a building and using the building to stop your car. It doesn’t tend to end well.

Coming down from a ski jump is like driving up a hill and allowing the hill to slow you down before you finally hit the brakes at a stoplight.

I am more interested in the learning curve, do they just put you at the top and tell you to fly and good luck?

To try and actually ELI5:

Ski jumps arent about who can jump the highest. If you watch, you’ll notice they stay just a little bit off the ground as they jump the whole way down!

Think about this : When you jump on flat ground, you go up and down, and stay where you landed. If you jump onto a slide, you’ll go down, but you’ll also go OUT toward the end of the slide. It makes you slide diagonally rather than straight down.

Put these together – not being very high off the ground and landing on a slide makes the fall damage very small.

Imagine you have a jetpack. You are flying vertical to the icy ground. You are wearing something to reduce friction even more. At that moment, it seems to you, like the ground isn’t moving closer to you. You are not falling. You could slowly fly lower and touch the ice. Glide on it. Or, the ground could slowly come up to you and touch you.

We have no problem with touching or landing on something that’s moving slowly towards us. And from that perspective with the jetpack, it would look and feel like you are slowly moving towards the ground.

So how do you achieve the same thing while ski jumping? You calculate the trajectory of a jump. And then you build the ground to go down at the same speed and height as the jumper. So imagine you jump, and if there was a cliff you would jump 100 meters. But now imagine someone builds a slope while you are jumping right under you, but not touching you. You will also fly 100 meters without touching the ground, but at every moment you are only centimeters away from the ground. It will look from your perspective like you are flying over the ground in a straight line. At any point you could put your feet down like you put your feet down from a stool.

And then you are just on a slope slowly stopping your fall, by getting slowly more and more horizontal.

If they landed on a flat surface, they would die instantly.

They don’t land on a flat surface. They land on a surface that is almost as angled as their angle of flight. The skiers speed directed at the landing surface is fairly small, like if you jumped a single flight fo stairs.