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Richard Berry · University of Virginia
First, I used the actual mass of the bag:
So, E = mgh --> 9kg x 9.8m/s2 x 2m = 176 J available potential energy.
Second, I tested some small LED lights for reading. The most suitable of those I had on hand was the Photon Micro-Light (use Google to investigate their web site). According to the manufacturers, this tiny light puts out 4.5 lumens. I found this to be adequate for reading 7-point Helvetica type in a book placed 1 m from the light; at that distance the useful circle of light is about 30 cm diameter. I opened the Photon Micro-Light and measured the voltage across the LED (3.56 V) and the current (39.4 mA), so it consumes 140 mW.
The GravityLight claims that it powers the LED at maximum brightness setting for 18 minutes. (The 30-minute figure applies to a lower output setting.) The Photon Micro-Light can illuminate a small room (3m x 4m x 2m) sufficiently well that you can move around without bumping into things.
So we expend our 176 J in 1080 seconds, which gives 162 mW.


GravityLight: лампочка за 5 долларов, работающая на гравитации