Gravity Light

Assignment: estimate the weight required for the GravityLight. List all assumptions, and determine whether this product seems viable. 

Things I Know
*LED bulb = 3.2V, 20mA
*Energy produced from the bag being pulled down powers an LED bulb for 30 minutes
*Gravity = 9.8m/s^{2
*An average person must be able to lift the weight above eye level, according to the picture:}

*The size of the bag is approximately 6" x 18" x 4" (length of a hand x height from mid-chest to forehead x approximate distance between his thumbs). 

      Taken from the video at 1:26.

Assumptions
*When I fly, I pack a suitcase that weighs 50 pounds. That suitcase is hard to carry up stairs, much less lift above my head. Therefore, I'm going to assume that the average person can comfortably lift no more than 35 pounds of dead weight. to eye level, but since the GravityLight bag is much smaller than my suitcase, I assume the weight is closer to 20-25 pounds.

Estimations
Approximate volume of bag:
          6" x 18" x 4" = 432 inches cubed
Approximate weight of 432 inches cubed of sand (source):
          25 lbs

Take away some weight from the area at the top of the bag that isn't filled:

          25 lbs - 3 lbs = 22 lbs

I estimate that the GravityLight requires about 22 pounds, or 20 pounds if we're rounding, to light an LED bulb for approximately 30 minutes.

This product seems like a viable alternative lighting source. Once production costs can be lowered to an ideal cost of under $5, it seems like a great light source, especially compared to kerosene lamps. 

One small issue is the LED life. LED's are cheap, much cheaper than batteries and solar panels. As Amy said, LED bulbs are cheap but not free. If the initial product came with 4 or 5 LED bulbs the length of the package life would be much longer.

Bonus: I checked out the Soccket. I'm surprised this technology hasn't pervaded every part of the world, considering soccer's (or futball's) global popularity.