# Homework Help: Mechanical energy/efficiency problem

1. May 1, 2010

### Flatshoe

1. The problem statement, all variables and given/known data

(i)Water flows over a dam at the rate 580kg/s and falls vertically 88m before striking the turbine blades. Calculate the speed of the water just before striking the turbine blades (SOLVED) and (ii) the rate at which mechanical energy is transferred to the turbine blades assuming 55% efficiency.

2. Relevant equations

Ek = 1/2mv^2 Mechanical efficiency = Work out/Work in

3. The attempt at a solution

v = 41.5 m/s

Mechanical efficiency = work out/work in

Work out = Ek1 - Ek2

Work in = Ek1 - Ek2/55%

Ok and I'm pretty much stuck. :(

Thanks in advance for any help guys!

2. May 1, 2010

### Squeezebox

This problem looks familiar did you post it on some other message board?

As you stated the mechanical energy is the ratio of the work out to the work in. What's the work in? The efficiency is 55% so, 55% of the work in is returned.

3. May 1, 2010

### Flatshoe

I'm having trouble understanding how to calculate the work in... :(

Also it says calculate the "rate", so how would the answer be expressed? J/s?

Edit: I've never posted this anywhere before.

4. May 1, 2010

### Squeezebox

Work is Joules/s. 580 kg/s fall off the dam. So work in is the energy associated with the mass that falls each second. What's the energy associated with the mass?

5. May 1, 2010

### Flatshoe

Kinetic energy? 1/2mv^2 = 1/2(580)(41.552)^2 = 500704 J = 500.704 KJ

Is that the right calculation for the work in?

6. May 1, 2010

### Squeezebox

Yes, now how does that relate to work in and finally, work out.

7. May 1, 2010

### Flatshoe

Thanks, I understand it now!