Author Topic: Airplane on a Conveyor Belt... (Read 32865 times)

AKIron · « **Reply #360 on:** February 03, 2008, 10:21:35 AM »

Both of you who responded got the hat riddle right.

That riddle and plane puzzle are similar in that with the hats you take into consideration what the two seeing guys saw and the fact that they couldn't solve the problem. Not so intuitive. With the plane/belt problem you must see that while the plane's powerplant applies thrust to the plane through it's engine mounts the belt is capable of applying an equal force in the opposite direction through the plane's wheels. Again, not easy to intuit.

SlapShot · « **Reply #361 on:** February 03, 2008, 10:25:37 AM »

Quote

Originally posted by AKIron
Here's an argument to say the guys that did this on TV did it wrong. Obviously they ran the belt at the same speed but opposite direction of the plane. If the original poser of the question intended for the belt to match the speed of the plane then why were "wheels" specifically mentioned instead of just saying the belt must match the speed of the plane?

I can't believe you guys are still debating this even after the "Mythbusters" busted it.

One thing you must have missed Iron ... Jamie said that as soon as the plane rolled ... HE GUNNED IT !!! ... meaning he went above the 25mph speed of the "tarp/conveyor" trying to make more of an effect ... and it had none ... the plane still took off.

The wheels are "freewheeling" and that is the key, and what everyone seems to ignore. Jamie could have had that tarp going 100mph (4 x the takeoff speed) and the plane would have rotated just the same.

The video of the belt sander with the wheel on it does show some force imparted on the wheels ... what lacks in the video is an "opposite" force ... such as an airplane engine ... so in essence ... that video really means nothing as far as this puzzle is concerned.

AKIron · « **Reply #362 on:** February 03, 2008, 10:35:06 AM »

Mythbuster's did not test the scenario we are debating here Slapshot. Read some of the hundreds of posts made in the various threads on this subject and that should become clear to you.

eskimo2 · « **Reply #363 on:** February 03, 2008, 01:31:07 PM »

The plane-speed question is a very good one. Why is it good? Because it fools so many people. Last year I initially read the wheel-speed question in the same way as the plane-speed question. I was surprised that it was even a question, that anyone wouldn’t intuitively get it the instant they read it. Honestly, I thought rabbidrabbit was either joking or completely clueless. As people chimed in, however, I realized that it really does fool many people. I even asked some bright people in RL, most of them were fooled and thought the plane wouldn’t fly. It is a good question because it separates people with different levels of understanding: those who DON’T get it even after reading thorough explanations, those who DO get it after reading thorough explanations and those who get it intuitively on their own.

The wheel-speed question is an even better one. Why is it better? Because it fools nearly everyone; the answer does not jump right out at you. Even after reading thorough explanations, very few people are capable of understanding the answer. It really does add a whole new tier of physics comprehension into the mix. No one intuitively saw the answer in the first reading. It took thorough discussions of the two paradoxes presented by this question for the answer to evolve. Most people do not understand the difference between the rotational energy absorption answer and the most basic level of misunderstanding where there’s a misunderstanding that the propeller turns the wheels.

AKIron · « **Reply #364 on:** February 03, 2008, 01:57:00 PM »

btw, I posted in that original thread as lukster.

Chairboy · « **Reply #365 on:** February 03, 2008, 02:10:00 PM »

Just for you, lukster.

Donzo · « **Reply #366 on:** February 03, 2008, 02:11:05 PM »

Quote

Originally posted by Chairboy

Just for you, lukster.

LMAO

AKIron · « **Reply #367 on:** February 03, 2008, 02:12:06 PM »

eskimo2 · « **Reply #368 on:** February 03, 2008, 02:14:22 PM »

That's great Chairboy!

clerick · « **Reply #369 on:** February 04, 2008, 01:53:41 AM »

Quote

Originally posted by hitech
I agree with you that bearing friction is a constant.

Only in a non-lubricated bearing. If there is any kind of liquid lubrication (grease, oil et.c.) the drag induced by the fluid will induce a force opposite the direction of rotation that increases as an inverse square to velocity. Granted that isn't true "friction" but it will act in the same direction.

Overlag · « **Reply #370 on:** February 04, 2008, 06:12:32 AM »

if the conveyor belt is stopping the plane move forward then surely there is no airspeed? (apart from the area near the prop)

hitech · « **Reply #371 on:** February 04, 2008, 09:56:27 AM »

clerick: Agreed, also with a rubber tire at higher speeds it becomes a none constant also.

But for general analysis , first pass it is generally treated as a constant.

HiTech

clerick · « **Reply #372 on:** February 04, 2008, 10:38:26 AM »

I hate when rubber isn't a constant. First she wants them, then not, then does....

VERTEX · « **Reply #373 on:** February 05, 2008, 01:13:44 PM »

Quote

Originally posted by eskimo2
That's an alternate question that has been simplified.

Here’s the original AH BBS question from last year:

Ok eskimo, but then;

By "wheel speed" do they mean the rpm of the wheel about the axle or wheel velocity? And if it is wheel velocity is it with respect to the conveyor, the ground, or the air.

Bear in mind that the rotation of the wheel does not give it "speed".

eskimo2 · « **Reply #374 on:** February 05, 2008, 03:36:47 PM »

Quote

Originally posted by VERTEX
Ok eskimo, but then;

By "wheel speed" do they mean the rpm of the wheel about the axle or wheel velocity? And if it is wheel velocity is it with respect to the conveyor, the ground, or the air.

Bear in mind that the rotation of the wheel does not give it "speed".

If someone said that he did a front wheels-brakes locked burnout and his rear tires were going 60 mph, pretty much anyone would assume that he meant the car was still, but where the rubber that meets the road part of the tire was spinning at 60 mph. Certainly you could argue that the wheels really weren’t moving though.

If someone said that an airport baggage pick-up conveyor travels at 5 mph, pretty much anyone would assume that top surface, where the bags ride, moves along at 5 mph. Certainly you could argue that the conveyor really isn’t moving at all because the lower layer of the conveyor travels at 5 mph in the opposite direction which cancels it all out.

Now in the plane-conveyor wheel-speed question, you could figure that the wheels are really moving at the same speed as the wing, rudder or flap indicator light. I think that it’s pretty reasonable to assume, however, that the term “…plane’s wheel speed” was specifically used to indicate where the rubber meets the conveyor.

I suppose that you could add a twist to any speed problem by including the Earth’s rotational speed, or the Earth’s orbiting speed around the sun, or the solar system’s speed in relation to the center of the Universe (big-bang).

Not that’s its wrong to think outside of the box and consider all possibilities; I’d considered this as well. But since the language of the question appears to be short and sweet instead of ultra detailed and specific and the question ends with the phrase, “similar to a treadmill”, I think it clear that we are talking about where the rubber meets the conveyor. Otherwise it would have been more succinct to just state that the control system that tracks the plane’s speed.