1. The problem statement, all variables and given/known data A tidal power plant generates 10 MW of electricity during a period of 12.5h. Assume that the water is collected behind a dam 6.0m high and is allowed to pass through a turbine twice to generate electricity, once as the tide comes in and once as the tide goes out. What mass of water flows through the turbine if the efficiency is 85%? Given: P=10MW= 10*10^6W t=12.5h= 45 000s h= 3m? g= 9.8 N/kg -the tide comes in twice, so I'll have to multiply the energy produced by two -the efficiency is 85%, so power will have to be multiplied by 0.85 -the max height given is 6m, but there's an example problem in this textbook where the max height of the reservoir is 6m again, but they use an average height of 3m? I tried using both 6m and 3m. Required: mass of water 2. Relevant equations P=Eg/t Eg=P*t Eg=m*g*h m=Eg/g*h 3. The attempt at a solution The problem seems simple enough but I can't seem to get the answer at the back of the book, which is 9.1*10^6 kg: efficiency is 85%, so 85% of the power produced should be P= 10*10^6W * 0.85 P= 8.5*10^6W then, solve for the gravitational energy produced Eg=P*t Eg=8.5*10^6W * 45 000s Eg=3.825*10^11J the tide pass through the turbine twice, so Eg=3.825*10^11J * 2 Eg=7.65*10^11J plug this in into the equation for gravitational energy and solve for mass m=Eg/g*h m=7.65*10^11J/ (9.8N/kg*3m) m=2.6*10^10kg so, that didnt work... let's try with h=6m instead m=Eg/g*h m=7.65*10^11J/ (9.8N/kg*6m) m=1.3*10^10kg nope :/.