Re: impulse/momentum

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

> 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.

Tom makes a very good point.  On the other hand, one *can* examine
the collision in the reference frame of the moving turbine blade in
which case the momentum change of the water merely serves to create a
force which is balanced by the load against which the turbine works.
In that frame the blade does indeed gain no energy.

We are then left with Jim Green's reasonable criticism that water
tends to be bad at "doing" elastic collisions which is answered by
Chuck Britton's reference to thrust reversers, which in essence
"sling" the fluid back in the opposite direction.

--
John Mallinckrodt        mailto:ajm@csupomona.edu
Cal Poly Pomona          http://www.csupomona.edu/~ajm