November

[marathondan]

The flat tyre analogy did indeed seem a bit out of place. But I was somewhat convinced when, at around 1:50, he says "any time I land in front of a forward moving body, I'm stopping it." This seems to make sense. All forces can be resolved into an x and y component (assuming I'm running in a perfectly straight line). When my foot strikes the ground, there is a y component which reacts to gravity and my downward momentum (stopping me falling down), and an x component which, at the point of impact, is indeed reacting to my forward momentum (slowing me down). The complication comes when my hip pivots, and as I push off, that x component is no longer a backward force but a forward force.

I haven't seen the rest of the film (Uncle David is next on my watch list, and I'm going to bed in a minute), so I don't know exactly where he's going with this. But the logical conclusion would be to footfall as far back as possible, resulting in a barely controlled stumble. If you could indeed control that, would it be more efficient? It sounds like there are a lot of factors at play, and simple analysis on a whiteboard isn't going to give a true answer. Somebody must have a computer model somewhere.

MLCM, I don't think you can increase your exposure to gravity without changing your mass (F=mg). Maybe I didn't get what you meant... But I agree that gravity cannot make you go forward. Only motive force or momentum (which must be topped up by motive force) can do that.