# Homework Help: Physics - Help with Power

1. Apr 12, 2005

### Nx2

Hi guys, I was doing some homework when I got stuck on this question I came upon. It goes as follows...

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

Ok, so I know the formula for power is P = W/t, so I changed W into F•d and now my equation is P = (F•d)/t. then I changed F into mg and got the equation P = (mg•d)/t.
Ok, now im not sure if what im doing is right but I don’t know where to go from here… they gave me velocity but I don’t know what to do with it. Like I subbed all my values in like the mass, gravity and distance but I don’t know what to do with the “75L/s and each litre is 1.00kg.”…. any help would be appreciated, thanks.

- Tu

2. Apr 12, 2005

### Gokul43201

Staff Emeritus
You have :
$$P = \frac {mgh}{t} = gh \cdot \frac {m}{t}$$

Does that help ?

3. Apr 12, 2005

### Nx2

ok... so... hmmm lol sorry i dont understand where that equation was derived from.... h is height right?

4. Apr 12, 2005

### Gokul43201

Staff Emeritus
Yes, h is the height. I just wrote what you had written and re-arranged the terms to bring out the factor (m/t). m/t is mass per unit time. Do you not know the value of this ?

5. Apr 12, 2005

### Nx2

sorry... yea... we never learned that b4.

6. Apr 12, 2005

### Gokul43201

Staff Emeritus
I'm not sure what you mean by this. Do you now understand how to solve the problem, or don't you ?

7. Apr 12, 2005

### Nx2

ooo... ok i tried it and i got the answer!... thnx alot i appreciate it. good help.

- Tu

8. Apr 12, 2005

### Nx2

yea i understand... thnx