Hmm.. I wonder if it's time for me to chime in yet. Any inconclusiveness still in the air? Any clarifications needed?
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Interesting physics/logic riddle
- Thread starter sahtt
- Start date
I hate that I've spent a lot of my idle brain time thinking about this... but I think I'm close to changing my stand on things. I kept thinking of other examples of thrust and the wheels turning but not being driven and other things. Here's my latest scenario: a box on wheels underwater at the bottom of a pool and I'm in scuba gear pushing the box along a treadmill. I put my hands on the box and swim toward the front of the treadmill and the wheels turn because the weight of the box has friction against the treadmill belt. But I think that I could swim with enough thrust that I could push the box off the conveyor belt, no matter how fast the treadmill turned.
Am I seeing it right? I think I finally got it. Another scenario that helped me get it is the space shuttle example, though not the first time it was mentioned because the poster didn't list the whole scenario. Or maybe I was just too dense to get that too!
J
Am I seeing it right? I think I finally got it. Another scenario that helped me get it is the space shuttle example, though not the first time it was mentioned because the poster didn't list the whole scenario. Or maybe I was just too dense to get that too!
J
You get it, but you could skip the scuba suit and just push the box on wheels forward on a treadmill if you like. :biggrin:
I agree with everything that you are saying. However if the belt is going fast enough then the drag on the wheels and bearings is going to be too much for the engine to overcome and the plane will not take off.
Regards,
Patrick
or... maybe the plane is going so fast that it would cause the bearings in the belt to burn out, then the plane would take off anyway... :wink:
Very true but I already covered that in one of my previous posts. When I said “So first we should agree that the conveyer belt or plane wont fail due to stress.”
Regards,
Patrick
Thus negating the quote below due to the fact that there would be no "drag" to cause such failures... :biggrin:
A wheel bearing does not have to fail to cause drag. All wheel bearings cause drag even when they are working perfectly.
Regards,
Patrick
Because we agree that the wording of the original puzzle is ambiguous. Please allow me to attempt to clean it up a bit.
A conveyer belt runs north south.
A plane sits on the belt.
The plane attempts to take off to the north. The belt operator runs the belt in a southerly direction at any speed she desires. Can she prevent the plane from taking off?
Regards,
Patrick
A conveyer belt runs north south.
A plane sits on the belt.
The plane attempts to take off to the north. The belt operator runs the belt in a southerly direction at any speed she desires. Can she prevent the plane from taking off?
Regards,
Patrick
Aircraft do not derive their forward thrust at their wheels. If a plane is traveling at 250mph on a conveyor belt, and the conveyor belt is moving at 250mph in the opposite direction, the aircraft is still moving at 250mph. The wheels are spinning at 500mph. That's the only difference. The wheels provide NO momentum at all! The friction caused by the wheels touching the conveyor belt isn't nearly significant enough to slow the craft down whatsoever.
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However if you wanted to play around with this, the universal answer is no.:biggrin: There is an elephant in the way. :tongue:
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However if you wanted to play around with this, the universal answer is no.:biggrin: There is an elephant in the way. :tongue:
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But don't you think the bearings on the conveyor would cause more drag than the bearings in the wheel of the plane?
BTW, we both agree that it can take off (see first page, and previous posts), so it's all just semantics now... :biggrin:
The drag on the bearings on the conveyer only matters if the conveyer does not have sufficient power to overcome that drag. However I think that it is safe to assume that if you were designing the belt you would make sure it had enough power to do the job.
If the belt is run with sufficient speed then the plane will not take off
Regards,
Patrick
Aircraft do not derive their forward thrust at their wheels. If a plane is traveling at 250mph on a conveyor belt, and the conveyor belt is moving at 250mph in the opposite direction, the aircraft is still moving at 250mph. The wheels are spinning at 500mph. That's the only difference. The wheels provide NO momentum at all! The friction caused by the wheels touching the conveyor belt isn't nearly significant enough to slow the craft down whatsoever.
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Yes but you admit that there is some friction so if the belt was run fast enough it would balance the thrust of the plane and then it would not move let alone take off.
Regards,
Patrick
Yes, but according to the riddle, the conveyor will only go as fast (in the opposite direction) as the plane is travelling, ie doubling the speed that the wheel is rotating. Most airplanes are airborne at well under 150 miles per hour, so do you think the friction on a wheel spinning at 300 miles per hour is enough to offset the thrust of the aircraft's engines?
And, the riddle clearly states that the plane is moving....
"The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)."
By these words alone the riddle says that the plane moves, not just the wheels. If the plane didn't move at all, why would the conveyor belt move? If the plane moves, it can fly.
"The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)."
By these words alone the riddle says that the plane moves, not just the wheels. If the plane didn't move at all, why would the conveyor belt move? If the plane moves, it can fly.
We have already agreed that. But if you go back further up the thread you will see that the puzzle is badly worded so that depending on how “the plane speed” is defined the plane will either take off or not take off.
Regards,
Patrick
This is a good question but I think we already covered it. It’s like if you say there is a man on a tread mill and he is running in one direction and the tread mill is moving in the other direction. If you were on a tread mill running at 10 mph and someone said how fast are you moving would you say “I am not moving” ? perhaps you would if you were talking about movement relative to the TV you were watching but it would be just as valid for you to say “I am moving at 10 mph” if you were talking about how fast you were going relative to the belt. Also there is no reason why the plane could not move and then after a few seconds time lag the belt operator would get the belt up to speed to negate that movement.
Regards,
Patrick
There is no "speed" of the plane if the plane is standing still. Conversely, the treadmill cannot work against the plane's speed and it remains stationary if the plane stays in one place. No forces to propel the plane forward are exerted on the treadmill at all. It's all thrust, pushing on the air. The air (in the riddle) is operating independently of the treadmill, and thus the plane will move forward through that medium. The plane MUST move forward through the air for the wheels to start turning.....there is NO scenario in the riddle where the plane will be standing still with the wheels rotating, either forward or backward. It's impossible with no drive mechanism on the airplane to turn the wheels.
like I said... semantics!
True but those semantics allow the plane to take off or not take off depending on your interpretation of the puzzle.
As ShiftyBob has pointed out we are arguing over semantics.Forget about the puzzle for a moment. Can we agree that if the belt was moving fast enough it would prevent the plane from taking off?
Regards,
Patrick
The rules of the riddle do not allow infinite speed of the belt. It clearly states that the fastest the belt will go is the same speed (in the opposite direction) that the plane is travelling. No MORE, NO LESS. There are no "semantics" to this. There are only two ways to calculate the plane's speed.....airspeed and groundspeed. Groundspeed is not measured by how fast the wheels turn, but by how fast the plane moves relative to the ground. There isn't a plane in the world that has a speedometer connected to the wheels.
IF we move out side of the boundaries of the riddle, then sure you can spin the belt up to whatever speed you want to make the plane's tires fall off. Within this riddle, however, there is no wiggle room. The rules are quite clear.
No matter how fast the belt moves the plane will still take off. Think of an NSX with a free spinning wheel/tire mounted right under the middle of the car and a treadmill 2 foot wide, infinate in lenght, spinning that center wheel as fast as it can. The rear tires of the NSX are still planted on the road. Hit the gas the NSX will move forward. The wheels of a plane have no more to do with forward motion than the center tire under the NSX. They are two seperate systems.
edit...Now if the treadmill is going forward with the plane at 2x the speed of the plane will the plane take off 2x faster?
Please read my post #89 in this thread and let me know what you think then.
Thanks,
Patrick
I went back through and read some more of your posts. You seem to go way outside the original question. The question is worded in a very trickey way. The wheels of the plane have nothing to do with forward motion.
What someone needs to do here is get a 1:18 car. Attach a model rocket engine to the car. Then get the car on a treadmill that is spinning fast enough that the car wheels are turning but the car is staying in place, then light the rocket engine. Record the whole thing and post the video in this thread.
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