Energy/power A levels physics

1. Feb 13, 2014

yan_plusheartz

Relevant formula
: p(out) / p (in) x 100 percent = efficiency
My question

My approach

But the right answer is 650kW

2. Feb 13, 2014

BvU

You use P = F x v but you write $\rho$ x Area x v3. Is that the same ?

3. Feb 13, 2014

yan_plusheartz

Yeah I understand that it is not exactly the same. But my problem is , I don't have the value for F and v . And the only formula I know for power is force x velocity. So I did it by using the values given to derive a unit which is the same as power. But I can't seem to get the right answer. and I'm not sure whether my approach is correct or not. Can you teach me the correct method?

anyhow, thank you for taking your time to try to solve my question:)

4. Feb 13, 2014

BvU

Yes I can. You know that power is energy / time ?
How much energy does the wind bring in /time ?

5. Feb 13, 2014

yan_plusheartz

If that case, the wind energy is 1/2 m (10)*2 am I right? Problem is , I don't have the value of mass and time :(

6. Feb 13, 2014

BvU

That is correct. And they give you the area and the wind speed.