## Sunday, October 09, 2011

### XO-1.75 seismograph A seismograph program which uses the accelerometer of the OLPC XO-1.75 laptop

The x,y,z accelerometer readings are saved in boxes x,y,z. Action 1 computes long term averages for these readings, (boxes a,b,c), 5% of the current reading is added to 95% of the long term average to compute a new long term average.

Action 2 determines a threshold of random noise which is ignored. It sums the squares of the deviation from average (x-a)^2 + (y-b)^2 + (z-c)^2 . If this figure exceeds 10, the screen is turned red to indicate that an event is occuring.

Action 3 prints the sum of the squares of the deviations and the x,y,z deviations.

Project source

Challenges:
• When an event occurs, graph the 3 channels (a)
• Save the 3 channels events with a time stamp for exporting into a spreadsheet (b), (c)
• Set up a network of laptops, an event is considered to have occured if all register it. (Use turtle position to share data)

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## Tuesday, October 04, 2011

### How low can you see that the earth is curved

At how low an altitude does the earth look like a disk, in other words, from how low does a line from the horizon to your eye trace out the surface of a cone (with angle a shown from the centre) ?

Surprisingly low. The depression of the horizon below level is noticable from an altitude as low as 218m. (Lysterfield Lake trig point 37°56'31" S 145°16'06" E, Elevation 218m) a = sin (a) for small a

a = 0.7/147 = 0.00476

r = 2h/a^2

r = 2 *218/(0.00476^2)

r = 19,000 km

Compared with the actual value of 6,371 km

Notes

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