Remember the headline: ENERGY IS NEVER CREATED OR DESTROYED
The amount of energy before and after are always equal. All we ever do with energy is move it around from one place to another.
a). A crane can't create energy. Lifting the same rock in 20 different ways always takes the <u><em>same amount of work</em></u>. It doesn't matter whether one person picks the rock straight up, or 50 people get around it and lift it, or roll it up a ramp, or lift it with 16 pulleys and a mile of rope, or use a giant steam crane.
You want to lift a 2200N weight up 25m, you're going to have to supply
(2200N) x (25m) = <em>55,000 Joules</em> of work.
c). YOU put out 55,000 Joules of energy. It had to GO someplace. Where is it now ? ===> It's the potential energy the rock has now, from being 25m higher than it was before. That <em>55,000 Joules</em> is NOW the potential energy of the rock.
No energy was created or destroyed. It just got moved around.
55,000 Joules of energy began as nuclear energy in the core of the sun. Solar radiation carried it to the Earth. Plants absorbed it, and stored it as chemical energy. You ... or a cow that you ate later ... ate the plants and took the chemical energy. One way or the other, the chemical energy got stored in your blood and fat. When you needed to put it out somewhere, you moved it into your muscles, and they converted it into mechanical energy. Then you used the mechanical energy to exert forces. Today, you used the original 55,000 joules to lift the flagstone, and NOW that energy is in the flagstone, 25 meters up off the ground !
