When we ski, we cant just look at the x-axis...becuase at the point our speed in the X direction is 0, our skis are in the fall-line, and our speed in Y-direction is now at its maximum...and correspondingly our kinetic energy is staying constant, its not getting transferred into potential as you believe....at least not 100% kentic to 100% potential like a trampoline. Some energy is lost to bend the ski, it has to...but it comes back...so where is the net gain you claim?
I was hoping to make things simple by focusing on the x-axis. If we can increase speed in either component, then we can increase speed overall. With discussions around ski rebound, I wanted to focus on the x-axis because that's where most of it happens. There's no need to bring in the Y-axis into the discussion. I know its not 100% transfer...doesn't need to be.
As for we can "push" to create tigher turns?
We resist forces with our legs...that is about it. Try standing on a bathroom scale - lets say you weight 200lbs...can you push with your legs so the scale reads 220lbs? No.
semantics. That's why I said "push" in quotes. I'm talking about resisting forces. It feels like a push with your legs. And btw, you can make the scale temporarily read higher than your weight if you pushed off it.
Have you expained my old school mountain bike riddle yet?
A bike turns with combination of tipping and the front wheel being able to steer...it's very different from a ski. In any case, you could get chucked anywhere in the turn depending on what you did wrong. Not sure where you're going with it, but I think it just adds complexity to the discussion as we have to take into account complicated bike mechanics as well.
Oh I still believe it. I just meant I argued it in a thread one time only (once). Keep guessing.
Zenny, I'm confused...I thought you're arguing that you don't believe you can accelerate out of a turn. But you're asking why a person might put their COM behind their BOS to generate more speed?
There is nothing like acceleration only in the X direction. The acceleration from the skis is perpendicular to the the skis. Some of the energy is released in the negative x direction (good) and some in the negative y direction (bad). If you release completely in the fall line you are going straight down.
I know...I'm not talking about accelerating only in the x-direction...I'm talking about looking only at the x component of the acceleration force.
ok lets try the turn phases and power inputs/outputs....still somewhat simplified because listing everything would be incredibly complicated.
Before we begin though, I just want to make sure everyone understands exactly what centripetal force is (I know at least Jamt does), but in case you missed out on HS physics class. Centripetal force is a force that acts perpendicular to the direction of travel, always inwards towards the center of a circle. Think earth going around the sun...the sun's gravity is the centripetal force that's always towards the sun. The earth orbits but never falls in because the earth has momentum, even though the force is always directly in the direction towards the sun. It is a force that turns us without slowing us down or speeding us up in a turn.
- phase 1 - transition to right before fall line: movements: legs extend to cause divergence of COM and skis. The COM is also pushed in a direction down the fall line as well (upside down position). power inputs: leg muscles, gravity. power output: skis dig into snow and centripetal force is created to turn the skis. Skis start bending as a result. Gravity contributes to some speed increase and helping accelerate the COM down the hill.
- phase 2 - fall line: movements: drop COM inside of turn to stay balanced against forces generated as a result of the turn. power input: leg muscles, gravity. power output: skis against snow creates centripetal forces to turn the ski/skier. Skis reach max bend as a result. Gravity has most effect on forward momentum of skis and skier.
- phase 3 - after fall line to transition: movements: legs relax to allow convergence of COM and skis. The retraction move isn't really a muscle move...it's the result of allowing the convergence of COM and ski to happen as the COM passes over the BOS. Power inputs: gravity, ski rebound. Power output: Skis still on edge creates centripetal force that keeps the ski turning. Meanwhile, the leg muscles have relaxed so the centripetal force acting on the COM goes away. This allows the COM to exit the old turn and enter the new turn. However! there's the case of the ski rebound. It is in the direction of the centripetal force (perpendicular to direction of travel). This force can be used to help the skis finish the turn while the COM starts moving into the new turn.
What's funny is after this exercise, it's even more clear that the direction of the rebound force being perpendicular to the ski is actually exactly what we need. We need it to be a centripetal force so that it can continue turning the skis even after we have reduced the centripetal force created by the leg muscles. It doesn't actually matter where in the turn this release happens..even if it's across the hill. Additionally, it actually doesn't matter if this force is released quickly or slowly because it generally adds on to the centripetal force to turn the ski after the leg muscles have stopped pushing the skis against the snow.
So why is it faster? cause it allows us to project the COM into the new turn slightly earlier than we would be able to otherwise. I don't need to wait the extra split second for the skis to finish its turn before projecting my COM..I can start projecting the COM and let the rebound of the ski help create the centripetal force to finish the ski's turn.
So I'm moving away from how much it speeds up the ski...rather it helps turn the ski and allows convergence to be quicker. The end result is faster!