Search this on youtube : <span>Photosynthesis Equation Song - Biology Lesson
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haha this might help.
When you drop a ball, it is dropped DOWNWARDS so the number is represented by a negative number. If you toss the ball up it will be a positive number.
To answer your question:
When an object is dropped downwards there will be a negative number since the object is falling downwards.
I may be wrong but i think that they are
1. turbines are added under the water and when the movement of water effects that is uses power to gather electricity
2. there are big barries in the water that spin the water around and then that in turn creates energy
