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Need a bit of help with a physics problem.

N

Newfie

Senior member
This is not really a "Highly Technical" question, but im not sure where else to post =P.

Here is my problem:

Water is pumped up to a water tower, which is 92.0m high. The flow rate up to the top of the tower is 75 L/s and each liter of water has a mass of 1.00 kg. What power is required to keep up this flow rate to the tower?

I am having trouble solving this problem, any tips?
 
im interested in how you solved it. I should know, though 🙁 Im thinking energy conservation using PE = mgh, but then you also have to consider the fact that the water is moving, so im not sure.

Alternatively, you can work it out using pressure, assume an area for the pipe you're using, then calculate the necessary speed, and how hard one would have to push to maintain a flowrate of 75 L/s for that pressure. The last bit is the tricky part, maybe it would just be F x D, but you also have to accelerate the water in the first place, so im not sure.
 
Originally posted by: Seer
im interested in how you solved it. I should know, though 🙁 Im thinking energy conservation using PE = mgh, but then you also have to consider the fact that the water is moving, so im not sure.

Alternatively, you can work it out using pressure, assume an area for the pipe you're using, then calculate the necessary speed, and how hard one would have to push to maintain a flowrate of 75 L/s for that pressure. The last bit is the tricky part, maybe it would just be F x D, but you also have to accelerate the water in the first place, so im not sure.
Click to expand...
This would be easily solvable using Bernoulli's equation for inviscid flow. That's the simplest approach, IMO.
 
I get the impression that this question is merely about potential energy.

To perform a flow analysis you need the fluid velocity or some way to derive it. Neither is provided in the question.
 
Originally posted by: Mark R
I get the impression that this question is merely about potential energy.

To perform a flow analysis you need the fluid velocity or some way to derive it. Neither is provided in the question.
Click to expand...
Ah, you're right. I thought it was m/s, not L/s. Without the velocity, this problem is only solvable in a textbook sense (i.e. there is no practical solution). One can calculate the rate of energy consumption in an ideal system simply by calculating the rate of change of potential energy, but without knowing the losses in the system, you can't predict the true power requirements for pumping purposes.
 
Power (in watts) = rho * g * Q * H

rho = density of water (kg/m³)
g = acceleration due to gravity (m/s²)
Q = flow rate (m³/s)
H = height (m)
 
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