Re: impulse/momentum

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 09:56 AM 12/12/2003, Tom Manz,  you wrote:
> The speed of water is v before and after the water hits the blade. This
> simply means the water changes direction (velocity), but not speed like a
> tennis ball bouncing off a wall.
>
> The kinetic energy of the water would be the same before and after the
> collision. By conservation of energy, this means the turbine does not move.
> By conservation of momentum, the momentum change of the turbine plust the
> momentum change of the water is zero. This means the turbine does not have a
> zero change of momentum.
>
> The only way both of these conditions can be satisfied is if the turbine
> mass approached infinite in comparison to the water mass. So there is
> something unphysical about these constraints. Would one really build a
> turbine which turned this slowly? I think not.

There is something unnatural about the propensity for modeling
infinite  masses. If a turbine wheel is locked, the conditions are satisfied.
If a turbine wheel is unlocked, then the startup phase is also approximated
by the model.
I therefore conclude that the sentences
 "the momentum change of the turbine plus the momentum change of the water
is zero. This means the turbine does not have a zero change of momentum" .....
are true and false, respectively.

Brian W



Brian Whatcott    Altus OK    Eureka!