Plane on a Treadmill

[Snow]

Here's another follow up teaser:

Now suppose the planes engines were off and that it was loaded with cargo in the front of the plane. The planes captain takes all the cargo and moves it to the very back of the plane. An observer standing next to the treadmill watchs. Does the observer see the plane move?

The way I see it is the cargo has to achieve some sort of velocity to get from the front to the back. If the cargo were pushed (frictionless) from the front of the plane at a certain velocity and then had a nonelastic collision with the back wall of the plane, the plane would have a force pushing it forward meaning that if it were on a runway it would move forward then stop (net kinetic energy gain = 0, net potential energy gain = 0).

But instead of sitting on a runway it's sitting on a treadmill that moves in the opposite direction of the plane. However we've previously determined that any motion of the treadmill has zero force on the plane because the plane still uses ball bearing wheels. So the only forces on the plane are pushing it forward as the cargo heads to the back and then pushing it back as the cargo hits the back of the plane meaning that the plane is displaced relative to the observer, starting still and ending still.

That's sort of right but wasnt quite the answer I was looking for. You dont need a big nonelastic collision with the back wall of the plane. In fact you could pick up the cargo, load it on a pushcart that has frictionless wheels / bearings etc, push the cart across the plane, lift the cargo off and gently place it back on the ground without touching the back wall of the craft or another box. Regardless of how you get it there (assuming no engery is produced like heat, etc), the best way in my opinion to think of it is the following: there are no external forces working on the plane + everything inside it. Hence the center of mass of the plane system does not move. By moving the cargo inside the plane, the rest of the plane must move to adjust such that the center of mass of the plane + everything inside it is unchanged. Hence, yes the plane appears to move relative to the outside observer.

[Androk]

Ah, but a FBD by itself may indicate motion, but it does not exactly mean that the object has to have movement relative to the system. In your description, assuming we changed it from a plane to a simple block on a treadmill, the force of "thrust" would create acceleration and movement in the block, which is then transferred 100% to the treadmill, meaning the treadmill would accelerate relative to the force. The block experiences no net movement.

I didn't know how jet engines worked exactly, but I figured it was a force applied against the pressure of the air right behind the engine, sorta like if you attach a balloon full of air to a straw with a thread through it, the balloon will move opposite of the direction the air inside is moving due to differences in pressure. Hence why I asked how the engine worked. If I had thought of it as moving in regards to how a propeller plane would move then I would have had a different answer.

Wrong. The treadmill has NO way of cancelling out the thrust. If the treadmill accelerates then the wheels accelerate. The wheels and the treadmill will ALWAYS cancel eachother out in a frictionless bearing environment. Which means that any force exerted on it will cause motion.

For some reason you're thinking that the thrust is some how providing torque to the wheels which is then causing the plane to move.

[Selene]

lol, the plane would crash and all would die in a fire ball of DOOM!
any way i know im not very smart on things like this, but to me if their both moving at the same speed, their would be no air flow to cause lift.
so the plane would not take off, it might at some point move off the mill and then would take flight.

[Yume]

amg

http://www.mercurynews.com/food/ci_7320 ... ck_check=1

for those who tl;dr'd it: mythbusters is testing it for their december 12th episode

[shiv_]

Last night's mythbusters was fucking awesome.

Put super hero myths to the test

[Lyion]

fuck i missed it, because all girls were dressed like sluts oh ya.

[futureal]

fuck i missed it, because all girls were dressed like sluts oh ya.

It was a day ending in Y?

[rankil]

to stir up the debate. The plane will take off. Take it from the 3 house of wow members that agree (jut/lemmi/me). I didn't even have to FBD it. You have a dumb question to start. Mainly you can tell it's a trick question to start. They want to you say it won't take off, but finding out why the plane will take off is the trick.

Now, the FBD is correct. But for the plane to not take off, it would need to use the wheels to power itself forward. With the treadmill moving, the wheels will move extra super fast. Now, in theory you might get something somewhere that could have some massive friction (but really the wheels would probably blow up first before the plane would take off)

[manx]

a. the tires will burn off and it will shoot off the back of the treadmill

b. the tires can handle unthinkable amounts of wear and tare and it will roll off the front of the treadmill.

since the wheela are working against the treadmill and the engines are working against nothing but air, wouldnt the wheels reach like a rotation lvl of infinity and then wouldn't the plane shoot off the front of the treadmill?

*edit- just thought about it a bit more, it would look just like a plane taking off on a solid surface speed wise.... the only way to keep the plane stable would involve the use of a wind tunnel to counteract the engines or whatever, which isnt part of the question... so my final answer is no, the this equation cannot exist in a perfect world! I think snovv pointed out that this equation would be much more interesting if the plane was propelled by its wheels.

you mean this?

[Yume]

http://mythbusters-wiki.discovery.com/p ... Plane+Hour

the segment was delayed but you can watch a couple video previews on the linked page. looks pretty kickass that they're doing it full-scale. New air date of Jan 30th

[Seph]

thats a pretty neat way of building a big ass treadmill, I guess a tarp dragged behind a truck will certainly do the trick.

[Yume]

Episode finally aired last night, hosting a crappy low-res version that I found on TPB: http://www.aenigma-guild.com/mythbuster ... f-3030.wmv

spoiler: it took off.