-- My mass is roughly, let's say, 91.8 kilograms.
-- The 2nd floor of my house is, say, 5 meters high.
So, when I lift myself from the first to the 2nd floor, I do enough work to increase my gravitational potential energy by
PE = (mass) x (gravity) x (height)
PE = (91.8 kg) x (9.8 m/s²) x (5 m)
PE = 4,500 Joules
It takes me, say, 10 seconds to make the climb, so the power is
P = (work done) / (time to do the work)
P = (4,500 Joules) / (10 seconds)
P = 450 watts (about 0.6 Horsepower)
This is enough power to keep four 100-W light bulbs and one 50-watter glowing at normal brightness.
But we have some work to do with the direction of the energy transfer.
When I stand at the bottom of the stairs and look up at the 2nd floor, I have a choice: I can either climb the stairs to the 2nd floor, OR I can pedal my generator-bike and light some lightbulbs. NOT BOTH.
I have energy in my blood and my muscles, which I can use to do work:
-- If I use the energy to lift my mass to the 2nd floor, then the energy goes into the mass and becomes gravitational potential energy.
-- If I convert my muscle energy somehow to electrical energy and light some bulbs with it, then it goes to the bulbs and nowhere else. I can't BOTH lift mass, give it potential energy, AND light bulbs.
I can use my energy to climb stairs, OR chop wood, OR pump water, OR climb a ladder, OR turn an electrical generator, OR play with my dogs, OR run around the block, OR let my brain use the energy to solve some math problems. Or I can distribute my energy, some here and some there, to do many things.
But I CAN'T use the SAME energy to do more than one thing.
