# Initial velocity

1. Feb 26, 2016

### goldfish9776

1. The problem statement, all variables and given/known data
how to know that the nozzle increases the velocity of water by 6 times? it's not given .... and no data for initial velocty given

2. Relevant equations

3. The attempt at a solution

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2. Feb 26, 2016

### SteamKing

Staff Emeritus
Did you look at the start of the problem calculations given in Example 5-1 at the bottom of the first image?

That's the one where the hose is being used to fill a bucket of a certain size in a certain amount of time. You can work out the velocity of the flow from the information given in the example. The second image just sums up the calculations from the first image.

3. Feb 26, 2016

### goldfish9776

yes, but i still cant understand the velocity increases by 6 times...

4. Feb 26, 2016

### haruspex

What is the ratio between the hose diameter and the nozzle diameter?
What is the ratio of their cross-sectional areas?
Since the volume flow rate must be the same for both, what does that tell you about the ratio of the linear speeds?

5. Feb 26, 2016

### SteamKing

Staff Emeritus
You've got a nozzle on the end of the hose. The water flowing through the hose is incompressible, so whatever amount goes in one end of the hose must come out the other end, in the same amount of time. You are also given the diameter of the hose and the diameter of the exit of the nozzle.

The flow rate through the hose Q = Area of the hose × velocity of the water. Since Qin = Qout and you can calculate Ain and Aout for the hose given the indicated sizes, then the only thing which remains is to calculate Vin and Vout. You are told how long it takes to fill a bucket with a certain volume of water, and this gives you Qin.

6. Feb 26, 2016

### goldfish9776

2 /0.8 = 2.5
1 / (0.4^2) = 6.25
so , the ans is It's 6.25 times , not 6 times?

7. Feb 26, 2016

### haruspex

Yes. I don't think they intended the 6 to be exact.