SF Beta

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SF 2009.1.21

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Marin Avenue (Berkeley)

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On Saturday, I got an email from Tim telling letting me know that Murphy's Spring Classics was going to start up Marin Ave. I'd missed that. Murphy announces his courses the day before, and indeed it was already well into the night before when the course announcement went out. I'd skipped over the details, jumping straight to middle and end games. Marin? I laughed at the irony, because not long before when he'd told me he was choosing to repeat the Nifty Twn Fifty which we'd done together last year versus selecting Murphy Mack's Stage Mullett two-day, I responded I didn't think I had it in me to ride Marin Ave two years in succession. Well, so much for that. I was committed, so I went downstairs to swap my 11-26 cassette for my Recon 12-27. The name is easily overlooked: "Marin Ave" fails to strike the same impression as, for example, "Redwood Gulch" or "China Grade". But among those in the know, Marin Ave is infamous i...
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Strava Suffer Score decoded

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Strava recently debuted its "Suffer Score", which attempts to quantify in one way how hard an activity was. Suffer score, they claim, is based on heartrate: ride further or harder and suffer score is higher. Suffer score is a good move for Strava. Strava's market segment is informal competition. Other web sites have logged how far people ride, and where, but Strava really locked into the demand for competition: competition via social networking. To date these rankings have been based primarily on speed over "segments": routes defined by users, for example on roads or trails, typically up climbs. They then added contests for volume: most miles ridden in a week and most feet climbed, for example. But with Suffer Score, they combine the two: a metric combining both quantity and quantity. Why's this important? Of course not everyone can climb fast enough to compete for KOMs on popular climbs. But a heartrate-based metric levels the playing field to ...
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hummingbird feeder physics

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I couldn't figure out how the hummingbird feeder worked. Why didn't it overflow? Forces need to balance, of course. Neglecting surface tension, there liquid level is higher in the inner reservoir than in the feeding chamber, so there must be a corresponding pressure difference. Suppose the pressure in the inner chamber were zero. Then the column height difference would need to be atmospheric pressure / (density of liquid × gravity). But this is over 9 meters! Obviously the height difference is only approximately 1% of this. So the pressure difference inside versus outside is only approximately 1%. The inside is only slightly below atmosphere. So air is getting in. How? Does it diffuse through the liquid? If this were the dominant mechanism, it wouldn't take long for the pressure inside to go from 99% to 99.3%, for example, which should be plenty to push the column of liquid down in the inside chamber and thus push liquid out through the holes. It would over...
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