Which formula to use?

[BobbyJones]

I have a question about a water wheel. Water is falling 80m from a reseviour onto the blades of the wheel. Assuming no losses and initial velocity is zero. Am I right in thinking the velocity of the water when it hits the wheel will be:

v = ut + 0.5at^2

v = 0 + 0.5 x g x h^2

v = 0 + 0.5 x 9.81 x 6400

v = 31392 m/s

This seems abit high

Also calculate the mass of the water flowing per second required to generate 10 kW electrical power.


Would that be the kinetic energy formula

Ek = 0.5 x m x v^2

10000 = 0.5 x m x 31392^2

m = 2.03 x 10^-5 Kg's

Am I totally wrong because these figures dont look right, but there is so many formulas for velocity!!!

 

[Deleted member 4993]

I have a question about a water wheel. Water is falling 80m from a reseviour onto the blades of the wheel. Assuming no losses and initial velocity is zero. Am I right in thinking the velocity of the water when it hits the wheel will be:

v = ut + 0.5a t^2 <<< This is incorrect - assuming ususal definitions of the variables. From where did you get that formula?

v = 0 + 0.5 x g x h^2 <<< Whrere did h go?

v = 0 + 0.5 x 9.81 x 6400

v = 31392 m/s

This seems abit high

Also calculate the mass of the water flowing per second required to generate 10 kW electrical power.


Would that be the kinetic energy formula

Ek = 0.5 x m x v^2

10000 = 0.5 x m x 31392^2

m = 2.03 x 10^-5 Kg's

Am I totally wrong because these figures dont look right, but there is so many formulas for velocity!!!

The formula to use is:

v² = u² + 2*a*d

 

[BobbyJones]

v² = u² + 2*a*d

v² = 0 + 2*9.81*80

v² = 1569.2

v = 39.62 m/s


Therefore the mass of water flowing per second required for 10kW of power would be:

Ek = 0.5 x m x v^2

10000 = 0.5 x m x 39.62^2

m = 10000/784.87

m = 12.74 kg

Subhotosh, have I now got the velocity and the mass of water correct?