Re: [Phys-L] Textbook Errors re Waterjet Levitation

[brian whatcott <betwys1@sbcglobal.net>]

There is a for-profit online entity called Brilliant.org which offers 
daily teaser puzzles for free on Facebook.
I attempted to recapture their puzzle from several days ago - the one 
about water jet levitation. Sadly, reviewing prior week puzzles comes 
only with a subscription. The gist of the explanation was to analyze the 
water jet in two parts: the u-bend with its momentum doubling effect, 
and the exit converging nozzle with its momentum increasing effect. (Not 
mentioned was the floating water pump's depression in the water while 
carrying the rider aloft.)

Possibly a bridge too far: I half recall a simple undergraduate model?? 
used in Statistical mechanics to model air pressure on a container: that 
a gas molecule is ascribed a velocity related to its temperature, 
whereby when the molecule reflects off a container wall, the momentum 
effectively doubles??????? Like photon reflection space propulsion ???

Brian W

On 10/21/2019 6:33 PM, Scott Orshan via Phys-l wrote:
> In the context of this discussion, I just got the latest Flying 
> Magazine, and there's a letter from someone who is asking how 
> clamshell thrust reversers work.
>
> The question asks, "In modern jet aircraft, most have a reverse thrust 
> capacity. I can see how turning the exhaust direction in the nacelle 
> produces reverse thrust. But how does a clamshell reverser work? To 
> me, when the clamshell deploys, it forms a reverse cone behind and 
> unattached to the exhaust of the engine. The engine ... produces 
> forward thrust. The clamshell diverts those gases after they have left 
> the engine back in the opposite direction. My question: Why aren't 
> those two forces offsetting? Why does a clamshell reverser work when 
> the gases have already left the engine?"
>
> As JD likes to say, accounting for the forces is sometimes difficult. 
> Accounting for the momentum is often much simpler. This is a perfect 
> opportunity for JD or someone to write in and explain.
>
> The forces *are* offsetting, as any forces in opposite directions must 
> be. But the reversing force is greater than the pushing force.
>
> If you look at the engine+reverser (with a big airplane attached) as 
> the system, you only need to look at the momentum leaving that system, 
> which is in the forward direction, so the airplane is receiving a 
> backwards momentum increase. If you look at the engine+airplane and 
> reverser+airplane as separate systems, the engine exhaust has a 
> certain delta-V, but the reverser exhaust has a larger delta-V because 
> the gases had to change direction.
>
> It's the same thing we've been discussing with the waterjet.
>
>
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