I am wondering if this method is in use already somewhere, or if its possible to be used. For example, using the equation of gravitational potential energy a 100kg weight that freefalls 100 meters has 98,000 J of energy, which equals 24,000 watts after approx 4 seconds of freefall. Assuming I used the equation correctly, I'd like to know if any or all of those watts can be 'harvested' into usable electricity? I'm thinking that maybe a device, like the brakes on an electric car, could bring the weight to a controlled stop after the 4 seconds of freefall. If thats possible would the amount of time for the controlled stop effect the electricity produced? My guess is that even if all 24,000 watts of energy could theoretically be 'harvested' the rate of electricity made available would decrease as the time for the controlled stop increased. If the stop took 1 second, for example, then the rate would be 24,000 watts/second, but if the stop took 60 seconds the rate would be 400 watts/second. Is that right, or anywhere in the vicinity of being right?