# wwhhat?

Discussion in 'Physics & Math' started by slayerdeus, May 26, 2004.

1. ### slayerdeusRegistered Member

Messages:
15
An average family of four uses roughly 1200 liters (about 300 gallons) of water per day. (one liter = 1000 cm3.) How much depth would a lake lose per year if it uniformly covered an area of 43 square kilometers and supplied a local town with a population of 43000 people? Consider only population uses, and neglect evaporation, etc.

Where do I even start?

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3. ### ddovalaPi is exactly 3Registered Senior Member

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141
well... 1200 liters/four people = 300 liters/person

times 43000 would be 12900000 liters/day

1000 liters = 1 cubic meter : 12900000 liters/day = 12900 m^3/day

1000^3 m^3 = 1 km^3 : .0000129 km^3

1.29 *10^-5 / 49 km = 2.63 * 10^-7 = .263 m (meh, got kinda lost in the algebra)

but as a check... .263 m times 49000000 m^3 = 12887000 it checks

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5. ### MacMRegistered Senior Member

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10,104
slayerdeus,

1 liter is 1,000 cm^2 x 1 cm deep
1,200 liters is 1.2E6 cm^2 x 1 cm deep
43,000 families is 5.16E10 cm^2 x 1 cm deep per day

Error. Population is persons not families

1 Km = 1,000 m x 100 cm = 100,000 cm

1Km^2 = 1E10 cm^2
43 Km^2 = 4.3E11 cm^2

Use Rate/Supply = (5.16E10/4)/4.3E11 = 0.03 cm/day or 0.0118 inches/day or 84 days per inch or 4.3 inches/year.

Last edited: May 26, 2004
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7. ### jamesmLost in thoughtRegistered Senior Member

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300 l per person/day * 43000 = 12900000 l / 1000 = 12900 m^3/day

43000000m^2 to a depth of 1m = 43000000 m^3

43000000m^3 / 12900 m^3/day = 0.0003m/day

0.0003m/day * 365 = 0.1095m/year = 4.3 inches/year

OK, so I get the same correct answer as MacM, but it's so fun!

8. ### slayerdeusRegistered Member

Messages:
15
yeh Mac is right, thanks alot guys