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  #61  
Old 11-23-2005, 11:34 AM
IndieMatty IndieMatty is offline
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Join Date: Jun 2004
Location: Losing 4/8 Stud Player
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Default Re: think about this...

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doubtful, that fan doesn't look very powerful.

if it did move, it would move backwards.

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pretend it's powerful enough

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i have zero college training, but the forces should balance, no? no movement of the skateboard (assuming the sail and fan are secured to the board)

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I can't believe you're seriously answering this. It's plugged into a friggin outlet. It's not going anywhere. Holy [censored]!

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The question was "will it move?", not "how far will it move?"

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[img]/images/graemlins/crazy.gif[/img] I suck at smart kid threads. Im out of here.
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  #62  
Old 11-23-2005, 11:35 AM
Eurotrash Eurotrash is offline
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Default Re: think about this...

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I think now that your OP has been reworded, Ray, that I understand what the situation you're actually describing is

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[img]/images/graemlins/confused.gif[/img] I didn't reword the OP at all - I only generalized that it could be basically any kind of plane, and specified there is no wind.

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well, I should have said that when your original situation was reworded by somebody in the thread (I just revisited this thread after not checking it for a while.) somebody likened the wheels on the conveyor belt to a removal of friction, which now makes a lot more sense. I'm not sure why I didn't think of it that way initially
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  #63  
Old 11-23-2005, 11:43 AM
Patrick del Poker Grande Patrick del Poker Grande is offline
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Default Re: think about this...

I haven't read any of this, but I can tell just from the number of replies that there's some stupid [censored] going on. I have two questions.

First, are we assuming a no-slip condition between the wheels and the treadmill runway?

Second, is the treadmill runway itself at rest relative to ground, or is it somehow zooming a hundred miles an hour along the ground?
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  #64  
Old 11-23-2005, 11:45 AM
jon_1van jon_1van is offline
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Location: Silver Spring MD
Posts: 53
Default Re: think about this...

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when the fan is plugged in, what happens? will the skateboard move? provide reasoning!

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The fan pushes a bunch of air to the left. And hense the fan wants to more right. But since the paper doesn't catch all of the moving air's energy (air moving left because of fan) the system is unbalanced. The skateboard moves right.
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  #65  
Old 11-23-2005, 11:48 AM
Slow Play Ray Slow Play Ray is offline
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Join Date: Oct 2004
Location: Beantown
Posts: 527
Default Re: think about this...

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First, are we assuming a no-slip condition between the wheels and the treadmill?

Second, is the treadmill itself at rest relative to ground, or is it somehow zooming a hundred miles an hour along the ground?

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Yes - no slip, as well as perfect bearings all around.

The treadmill mechanism itself it not moving relative to the ground, the conveyor part is. Just picture a moving walkway like in an airport, but much bigger (and faster).
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  #66  
Old 11-23-2005, 11:50 AM
fnord_too fnord_too is offline
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Location: Norfolk, VA
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Default Re: think about this...

If it moves, it will be backwards since not all of the momentum from the air being forced forward will be imparted to the board.
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  #67  
Old 11-23-2005, 11:54 AM
Hiding Hiding is offline
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Posts: 99
Default Re: think about this...

Equal and opposite reactions, the pressure on the board and the pressure on the fan are equal, in an ideal situation, so no motion.

If the planes wheels can't slip and the conveyor structure is motionless, with respect to the plane and air flow then there is no lift.

If a truck driver has a load of chickens at a weigh station and beats on all the cages so the chickens are flying at the moment he is weighed, does the truck weigh less?
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  #68  
Old 11-23-2005, 11:54 AM
BOTW BOTW is offline
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Default Re: think about this...

A plane is stuck to the ground cuz gravity is pushing down on it.
To lift the plane, we need airspeed.
There is no wind so airspeed = ground speed.
To get ground speed, we need to move forward compared to the ground.
We apply thrust to the plane.
Relative to the wheels contact on the conveyor, the plane moves forward, so the wheels start spinning.
However, our conveyor spins beneath our wheels at the exact speed our wheels turn.
Hence, if the conveyor itself isn't moving, the plane isn't moving and we have no ground speed.
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  #69  
Old 11-23-2005, 11:56 AM
Patrick del Poker Grande Patrick del Poker Grande is offline
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Default Re: think about this...

The plane can take off. The wheels and runway will accelerate like a mofo as it does. The wheels have nothing to do with the lift. The wheels do nothing for the plane other than provide a minimum altitude relative to the ground.
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  #70  
Old 11-23-2005, 11:57 AM
fnord_too fnord_too is offline
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Join Date: May 2004
Location: Norfolk, VA
Posts: 672
Default Re: think about this...

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A plane is stuck to the ground cuz gravity is pushing down on it.
To lift the plane, we need airspeed.
There is no wind so airspeed = ground speed.
To get ground speed, we need to move forward compared to the ground.
We apply thrust to the plane.
Relative to the wheels contact on the conveyor, the plane moves forward, so the wheels start spinning.
However, our conveyor spins beneath our wheels at the exact speed our wheels turn.
Hence, if the conveyor itself isn't moving, the plane isn't moving and we have no ground speed.

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The props/jet are creating a force on the plan. How does the conveyor create an opposte force if the wheels don't skid? That is the crux of the problem.
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