Hmm.. I wonder if it's time for me to chime in yet. Any inconclusiveness still in the air? Any clarifications needed?
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 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
It took me a while to grasp this, that was my original thinking too, but the belt moving backwards at the same speed the plane is moving forwards does not keep the plane stationary. It would keep a car stationary, but not a plane.
If you put a boot on a cars wheels to prevent the wheels from turning, a car isn't going to go anywhere because the wheels are what provides momentum to a car, if you put a boot on a planes wheels, the plane is still going to go, it's just going to tear the wheels apart in the process. The wheels have no bearing on a planes movement, they're just there to make it easier for the plane to move forward by rolling.
If a plane is moving forward at 20 mph and you stick it on a treadmill moving backwards at 20mph, the plane is still going to continue moving forward at 20mph, but the wheels will spin at 40mph. The spinning wheels again have no bearing on the motion of the plane since they aren't what's causing the plane to move in the first place.
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:
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
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
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
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:
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.
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
like I said... semantics!
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.
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
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
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