Re: impulse/momentum

[Jack Uretsky <jlu@HEP.ANL.GOV>]

        Tom missed the word "stationary" in the statement.


On Fri, 12 Dec 2003, Tom Manz 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.
>
> ----- Original Message -----
> From: "Jim Green" <JMGreen@SISNA.COM>
> To: <PHYS-L@lists.nau.edu>
> Sent: Friday, December 12, 2003 2:23 AM
> Subject: Re: impulse/momentum
>
>
> > > This one is from Halliday Resnick and Walker, problem 10.17, which I
> > > quote:  "A stream of water impinges on a stationary 'dished' turbine
> > > blade.  The speed of the water is v, both before and after it strikes the
> > > curved surface of the blade, and the mass of water striking the blade per
> > > unit time is constant at the value u.
> >
> > "speed of the water is v, both before and after"  Wow. Very strange water.
> >
> >
> > Jim Green
> > mailto:JMGreen@sisna.com
> > http://users.sisna.com/jmgreen

--
"Don't push the river, it flows by itself"
                                Frederick Perls