# Downhill Physics Question



## PaoloSmythe (Aug 19, 2007)

tis an interesting question.

the ansa, is that air time = slow time. 

now the limitation on my explanation as to why this is, is that air resistence slows you down as though you're a sail... but why such fails to apply on the ground i do not know.

also, altho your board essentially creates warm friction to melt the snow and thus allow you to slide, this is still friction, so shouldn't this add a tiny bit to the slowness?

i am bamboozled!


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## kri$han (Feb 24, 2007)

well, the way I see it is that in both cases, air resistance will be constant, so you can't really consider that (whether you're flying through the air, or on the ground, air is still there).

I don't think there's a way to prove this, but my hypothesis is that the guy that stays tucked and rides all the way down will get to the bottom first. Simply because, if you were to straight-line a hill, you're pretty much accelerating down the hill constantly.

Now if one was 'jumping' (or ollie-ing, right? ) you'd be interrupting this constant down-hill acceleration by applying a force downwards and thrusting yourself upwards. Granted, when you are 'falling' from your jump, you'd be accelerating faster than if you were on the snow, but the constant thrusting upwards would keep limiting your speed, imo.

Honestly, the ONLY way to truely know is to test it, since there really is no formula or way you could prove this with theory.


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## nzboardlife (Mar 25, 2008)

horizontally(as you are a projectile) you will always both be traveling at the same speed, except the one hitting jumps gets slowed down by the vertical incline of the jump... i think.



^^^^ there acceleration is exactly the same at all times, 9.81 ms^-2 downwards


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## kri$han (Feb 24, 2007)

Of course their free fall acceleration is 9.81m/s^2, indeed, but you'd have to multiply that times a comonent of each riders weight (since they're not going down a 90degree vertical), subtract friction and air resisitance, and (like I said) one would be constantly accelerating, while the other intermittently accelerating upwards.


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## Guest (Sep 16, 2008)

edit: yeah what he said ^^^


im guessing the person who doesn't jump wins

the air friction doesnt matter but as you land from a jump some force vertical force is lost as you absorb the landing. also at the peak of the airborn parabola vertical acceleration is 0 where as if you dont jump you are constantly accelerating at a tangent to the force of gravity...? can someone who actually knows what they're talking about correct me


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## PaoloSmythe (Aug 19, 2007)

so i guess the momentum of going down hill, is reduced due to the 'upward' trajectory' caused by taking flight....

with points being A to B (top to bottom) any deviation from this, is to lengthen the 'straight line' which is the shortest and thus fastest way from point to point. so flight = greater distance.

also the arch of the airbourne flight path sees the upward momentum (lift off) being countered by the ever present force of gravity (to bring you back to earth).... a gravitational force which could be better used to get you to the 'finish line'. 

ie gravity has to work to bring you back to earth before it can continue to drag you to the bottom of the slope.

makes sense to me!


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## KIRKRIDER (Mar 18, 2008)

well the fact that everytime you land a jump you weight more ( mass) and dig into the snow a bit for the added mass will slow you down, because of the more friction you generate each landing.

If the jumper rider could "fly" down never touching the snow he/she will be faster.


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## Mooz (Aug 14, 2007)

pawlo said:


> well the fact that everytime you land a jump you weight more ( mass) and dig into the snow a bit for the added mass will slow you down, because of the more friction you generate each landing.
> 
> If the jumper rider could "fly" down never touching the snow he/she will be faster.


Epic physics fail. Your mass is constant. Your momentum changes when you jump and land as well as your directional force. MPD was right about air time though. You have two forces attempting to slow you down. Friction from board on snow and friction from air resistance. Also remember the shortest distance between two points is a straight line. Jumping creates an arc. You'll also dramatically increase wind resistance by going airborne. Sure you lose the friction from the snow but you're also removing your downward momentum in favor of an airborne momentum and arc. You're also losing that which you need most. Momentum from the angle of the mountain.

The fastest rider will be the one that stays on the ground tucked in tight. The "flying" rider (jesus that made my head hurt) is still slower because they lack anything to propell them downward aside from gravity and they still have to deal with the incredible amount of wind resistance. Gravity is up and down you still need momentum to traverse the hill unless it's a cliff. If it's a cliff all bets are out since they're both going to be dead on impact anyway.


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## Guest (Sep 20, 2008)

you bunch of eggheads..lol... I think common sense wins out here. Think of a BMX racer doing everything to AVOID catching air over jumps? less air equals more speed; This principal applies to downhills ski racers, bmx riders or a snowboarder racing gates.


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## Guest (Jun 19, 2009)

*Thinking outside the box*

I was watching the Red Bull jam session featuring Shaun White and T-Rice in NYC. They were going down a relatively short ramp and in order to build up enough speed they were being flung out onto the ramp and were hopping their way down.

It does help to hop. The reason is that when you're jumping on a slope, you are jumping at an angle that has both a vertical and a horizontal component. The vertical component is negligible because all the energy you put in will be returned by gravity. Net change in vertical potential energy will remain the same. The horizontal component will translate into the actual acceleration.

Also take into account that the non-jumper has a constant amount of friction from the snow. The jumper will spend part of that time in the air, and in general air friction is much lower than any friction you get from the snow you're riding.

However, jumping does cause the board to dig into the snow somewhat, which increases friction. So there is a balance between how much speed you gain from your air time plus the horizontal component of your jump, versus the increased friction you receive from your board while jumping and landing in the snow.

Every jump a snowboarder makes will add a constant amount of energy to his acceleration. However, as his velocity increases, the frictional forces from digging into the snow during inefficient jumping and landing will increase. At a certain point, the extra energy from the jump will not balance out the increased friction.

So bottom line... It helps to jump at first, but once you gain some speed, start cruising.

Thank you.


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## Guest (Jun 19, 2009)

oneplankawanka said:


> you bunch of eggheads..lol... I think common sense wins out here. Think of a BMX racer doing everything to AVOID catching air over jumps? less air equals more speed; This principal applies to downhills ski racers, bmx riders or a snowboarder racing gates.


It doesn't work for snowboarding. BMX bikers can accelerate when they're making contact with the ground by pedaling. That's why they try not to be in the air. Snowboarders rely solely on gravity for increasing their speed.


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## BoardTheSnow73 (Apr 25, 2009)

Mooz said:


> Epic physics fail.


:laugh::laughlaugh::laugh:

Agreed, although some definite progress was made toward the end. I knew there was a reason they made me take all those physics and fluid mechanics classes in school.

I would have to agree with jiggydancer that at slow speeds there is a good chance you could go faster while “hopping”. Air drag is proportional to the square of speed (so if you double your speed you get 4 times the wind resistance, 4 times the speed gives you 16 times wind resistance, etc.). So air drag is your main problem at high speeds, but at low speeds the other factors will play a larger role.

I don’t think that the “increased distance traveled due to jumping vs. straight line” is a valid argument against hopping (as long as you are jumping perpendicular to the slope) because you will also be increasing your total speed with the addition of a perpendicular velocity vector (you add the energy to do this extra work). 

Useful gravity does not have to do any more work on you to bring you back to the slope. Gravity’s pull on your mass can be broken into two components. One component of this force acts along the slope, and this is what makes you go faster. The other component acts perpendicular to the slope, and the slope resists with an equal force. This interaction results in friction, which only slows you down (Figure 1 attempts to show the components involved). So separating yourself from the slope removes the friction, which helps.

The problem comes from achieving separation. When jumping, you apply an extra force in addition to your own weight. When you land, an additional force is also required to stop your momentum. Both of these situations result in an increase in friction. They also result in you compressing the snow under you which will cause a larger amount of snow to be “plowed” out of the way by the nose. The plowing action creates a force opposite to your intended direction of acceleration (this could be eliminated if riding in icy conditions) (Figure 2 attempts to show this).

The problem with the landing is compounded by the fact that, for an instant after landing, you will not have water channeling under your base to reduce your friction coefficient. The force of friction equals the normal force times the friction coefficient, and you are increasing both while landing. On the other side, (I may be wrong about this) as you ride continuously, your base warms up some which makes it more efficient at melting the snow and reducing the friction coefficient. 

In the long run, air drag is a much greater problem than anything else so stay grounded, get low, get skinny, and get some spandex on.

I think the real question is: Do you want to be popping ollies at 60+ mph?


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## DC5R (Feb 21, 2008)

Holy fak!! I thought I logged into a NASA website for a second, LOL!!


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## Music Moves (Jan 23, 2009)

Lol at a lot of complication of the obvious... diagrams are nice though .


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## Guest (Jun 23, 2009)

Analysis Paralysis


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## Guest (Jun 23, 2009)

Lol... Way to go guys.

But yeah. Practical experience tells me. Hopping helps.

And at very slow speeds or on flat ground, it might help to Nollie.


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