Refer to question #1 on the link provided below: http://websites.rcc.edu/cheney/
ID: 1372395 • Letter: R
Question
Refer to question #1 on the link provided below:
http://websites.rcc.edu/cheney/files/2014/08/Physics-2A-Homework-7.pdf
In order to receive full credit on the problem these steps must be done:
1. Draw two pictures: one for the initial position of the object and one for the final position of the object.
2. Label all relevant information in each picture, including: the speed of the object, the height of the object the stretch of any spring in the picture.
4. Make a list of forces (other than gravity or springs) that act on the object while it moves from its initial to final position.
5. Write conservation of energy. Start with the form exactly as it is written in the notes above.
6. Replace each K with ½ mv2 . Replace each Ug with mgh. Replace each Us with ½ kx2 . Enter zeros where appropriate.
7. Replace Wother with a list of the work done by the “other” forces that act on the object. This list will typically include normal, friction, tension, etc. DO NOT include work by gravity or springs!!
8. Replace each “W” with the appropriate expression. If the force is not a special force, use the general expression for work given above.
9. Carefully inspect your equation (only one equation!) for known and unknown values. You should have only one unknown. Solve for it.
Answer is at the bottom of the sheet on the link I just want to know how to properly arrive to that answer.
Explanation / Answer
water entering the dam falls from the height = 180 meters, its potential energy will be converted into electrical energy
potential energy = mgh = m* 9.8 * 180 = 1764m Joules
Power = work done/time
here, time = 1 sec
so power = 1764m Watts
but, generator is 75 percent efficient, so, 75 percent of 1764m = 2.08 GW
1323m = 2.08 * 10^9
m = 15.72 * 10^5 Kg
this is the required mass of water.