# foot power

Alan Eliasen eliasen at mindspring.com
Thu Jan 27 21:54:16 EST 2011

```On 01/27/2011 04:08 PM, Sameer Verma wrote:
> Looking at this thread reminded me of clocks with weight pulleys. I
> saw one recently outside UCSF hospital.
> http://www.flickr.com/photos/revger/4381874187/
> The weight unwinds the pulley slowly. This is also seen on roasting
> jack (For those who attended the OLPC SF dinner/bbq, the Kleiders have
> this at their house)

All of these questions are easily answered with some basic physics.
E = m g h   will calculate how high you need to lift a mass to get a
certain amount of energy.  I wrote some analysis of a gravitational
system in the OLPC wiki:

http://wiki.laptop.org/go/Battery_and_power#Pulley_Power

I'll quote it here:

"The physics of such a gravitational potential energy system are
easily analyzed. Assuming the figures on this page of ~6 W draw are
reasonable, one would need to haul a 5 kg (11 pound) bucket to a height
of 440 meters (1445 feet) feet every hour, and that's assuming perfect
conversion of potential energy to electrical energy. The reality would
be probably more like 25% efficient, or less, so assume a 2 km-tall
tower. Not very realistic. Does the kid carry around and erect an
immense derrick of that height every time they need to charge?
Alternately, you wouldn't have to lift it as high if you lifted a larger
mass. If you only wanted to lift it, say, 3 meters into the nearest
strong tree, you could instead carry around an immense, sturdy bucket
capable of holding 734 kg (1620 lb) of sand or water, fill that full of
sand or water every time you stopped, rig a complex set of pulleys to
give sufficient mechanical advantage so that the bucket's weight doesn't
fling the child into the low stratosphere, and then haul it up those 3
meters into the tree every hour."

Again, I will note that the only way to make the XO reasonably
powerable by a child (or an adult) is to reduce its power consumption.

--
Alan Eliasen
eliasen at mindspring.com
http://futureboy.us/

```