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Old Sep 9th 2017, 10:08 PM   #1
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Computing Gravitational Potential Energy

I am taking a physics class for non-physics majors.

Q:
As water descends from the top of a tall hydroelectric dam, its gravitational potential energy is converted to electric energy. How much gravitational potential energy is released when 900 kg of water descends 220 m to the generators?

Going off the book,
Gravitational Potential Energy = (Mass)(Acceleration due to gravity)(height) or u = (m)(g)(h)

g = 9.8
m = 900
h = 220

(900)(9.8)(220) = 1,940,400 Joules or 1,940.4 kJ

It says this is incorrect. What am I missing?
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Old Sep 10th 2017, 12:50 AM   #2
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Your answer is correct ...or to be precise 1.9404MJ

Don't be surprised , text books , particularly in the US are full of errors and misleading and confusing questions ... it's planned to be that way by the people at the top , to keep people frustrated and stupid ..

See Charlotte Iserbyt 'The deliberate dumbing down of America"
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Old Sep 10th 2017, 01:38 AM   #3
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Although Physics is not your major, you obviously have an interest.

Yes your calculation is correct as far as it goes, but if all the potential energy of the water was converted to electricity than there would be nothing to cause the water to flow onwards at the bottom.

You might like to look through this (short) thread to understand a little further.

Mass flow rate of water in a hydraulic turbine
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Old Sep 11th 2017, 07:29 AM   #4
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Given that you've used a value for g that is good to only two digits of accuracy, the answer you gave is "over precise," meaning you have too many digits of accuracy for the data provided. In other words rather than 1940.4 kJ, see if the system accepts 1940 kJ instead.

If that doesn't work, try using a value of g=9.81 m/s^2 and see if the result is considered "right."

Last edited by ChipB; Sep 11th 2017 at 07:31 AM.
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