Puzzles,conundrums, riddles and thoughts?

[Azash]

Any more takers? It's not darkness or sleep.

Confusion?

 

[Eran of Arcadia]

Time?

A towel? No, that "dries as it gets wet".

 

[Azash]

How does a towel leave you in myster-

I know! It's [wiki]Towlie[/wiki]!

 

[Abaddon]

dejarvu?
xcl

 

[Sophie 378]

Is it Fog?

Yes. Ten characters.

 

[mdwh]

Okay, picking up the airplane-on-treadmill puzzle again, as I was directed here from the now-closed thread, I'm still confused/sceptical by this:

http://www.straightdope.com/columns/060203.html

the important thing about the airplane riddle is, that the conveyer belt will only affect the movement of the wheels, but not that of the plane.

But clearly it will - consider when the engines aren't on, the plane goes backwards. Unless you have frictionless wheels.

I think this is key to it:

But: Its not the wheels that generate the velocity. The velocity is instead generated by the engines, which pull against the air to create thrust - it doesn't matter what the conveyor does - its just spinning the wheels.

If a plane moves because the engines pull it through the air, or push against the air, then yes, it will move forward.

But a rocket on the other hand doesn't work like that - it works by Newton's third law, and the generated motion is not relative to the "air", "vacuum", it's relative to the plane itself.

So let's rephrased the question, with a rocket, in a vacuum - obviously we can't take off anymore, but will it move forwards relative to the ground? I say No.

So does the question change with a propeller or jet engine? With a propeller engine, I say yes, because it works by pulling itself through the air, hence the motion is relative to the air (although friction from the wheels could create additional drag...?)

But my impression was that jet engines worked via Newton's third law, similar to rockets? I thought the propellers on a jet engine are there to compress the air for combustion, and aren't responsible for the thrust...?