Imagine a plane is sitting on a treadmill

The belt needs to exert a constant force on the plane to exactly match the forward thrust from the engines.

The belt is not exerting any force on the plane since we are all making the assumption that the wheels are free-wheeling with negligible drag from bearings, etc.

OK, so you are saying that the only force acting on the plane (horizontally) is the thrust from the engines? Right, I think you've just proved the plane will accelerate forwards then. Nicely done.

if the plane can take off without considering the friction on the ground, I wonder why we need a runway?

Simply put, we need a runway because the plane needs to gather speed until the flaps are pushed down to give it the final pull-up to take off.

So it does matter that the wheels are on the ground and spinning at some spped, unlike some have said.

The issue is that the with a conveyor belt there, the speed cannot be achived to take off...

Thos who say that friction plays no role, must ask why we need runways...

On the other hand, if we are talking about choppers or VTOL machines, it is another story.

I don't know if I should be laughing, crying, or banging my head on the table.

Ahh, of course.. all three!

DC77, how about if the plane were on skis on ice, or on water? Would it still take off? What if the ice was moving backwards on a conveyor, would it still take off?

Wait a minute. Now you say that I support your argument by arguing the opposite point you made about friction?

huh

You need a runway because initially plane is standing still=no lift. If you can hold the plane in this case with a rope, then sure you don't need a runway. It starts moving and accelerates until its velocity is enough to produce sufficient lift to take it off the ground.

yes will still take off since there is friction. Ofcourse we need a longer runway...

If the ice was moving backwards, then if the speed is not achieved, then no...If the speed is acieved, yes.

You know that the lengh of the runway is designed to have a certain speed for take off...right?

The speed I am talking about is w.r.t. the absolute still ground.

Let's ask Airbus to make a demo system and prove it one way or another.

Sorry, but I am not wrong. The conveyor belt is keeping the plane standing still. Tell me how it will take off with no air flow?

It is not like a jet taking off on ice, ice does not propel the jet backwards at the same rate the thrusters are forcing it forward.

Mate, if I was you I'd suggest you forget about this thread and start another one about cars or something. I met Jenson Button the other day actually - nice bloke.

I think we need sGb27's persistency again here to persuade some people.

Look, either you want to include friction or not. If you don't then there is no way the conveyor can exert any force on the plane and it will easily take off. If you do then the conveyor needs to move with (very high) constant acceleration to counteract the force from the engines. I don't mind which solution you prefer.

Yes, but it says that the conveyor belt matches the speed of the plane, so whatever speed the plane is moving forward, the conveyor belt is keeping it still. The wheels do not matter, but they will not leave the conveyor belt all things being constant.

Once again, how? The wheels spinning backwards exert very little retarding force on the plane as it accelerates due to Newton's third law.

neither does a conveyor belt, because the wheels are free to spin.

If the brakes were on, different story. Or if drive was though the wheels, like a car. In either of those cases the wheels are not free to rotate backwards so motion would be retarded.

Wow that is just like soooooooooo freaky. How can the wheels be spinning faster than the ground they are in contact with??? That would be like impossible huh.

Well not really considering the normal situation is that the ground does 0mph and wheels do 50mph every time you get in the freaking car to work.

The flat-earthers are getting wittled down now, so we only have the ones that were dropped on their heads as babies to deal with now.

I think you are confusing "matches" with "counteracts".

It also depends how the question is worded. I am being devils advocate here. Since wheels on a jet are pretty much frictionless, the conveyor belt would have to move at an incredible speed to keep the plane still

I only met Tiff Needel and he was a bit of a co..

But saying that, I still like him for some reason.

I wish that were true, but each time I check the poll the percentage of no's is increasing

Doh!

Correct. The question says they don't though. So, as devil's advocate, are you convinced?

A bit???

I've met him several times.

Hooray!

Right so let me see here. You say "the conveyor belt is keeping it still" - so I will take that as plane travelling at 0mph.

But you also say "conveyor belt matches the speed of the plane" - so conveyor belt is now doing 0mph.

C'mon people. This is just getting embarassing.

All doubters look at this animation:

http://www.deviantart.com/deviation/44285447/

the wheels on top are those providing the force, and could just so easily be the jet engines of a 747. As the animation proves the wheels on the conveyor must merely spin twice as fast (easy since they're able to freewheel), to achieve the normal speed. As said before - to achieve an airspeed of 200kts the wheels would have an rpm equivalent to that which they would have with an airspeed of 400kts (thanks to the conveyor). The small amount of friction etc generated by the wheel bearings would mean the plane needed ever so slightly more time to achieve rotation speed, but it still would.

YES, dammit, YES!! That's the whole point. The wheels are moving at 400mp/h because the plane is moving forward at 200mp/h, powered by the thrust of the engines, and the conveyor belt is moving in the opposite direction at 200mp/h. The wheels are therefore moving at 400mp/h, the plane is moving forward at 200mp/h and will therefore take off. See the link below before digging yourself any deeper. Even Sin now appears to have accepted reality.


You are so wrong it's funny. If you'd bothered to read the thread before charging in, you would have seen the following link, which explains in nice simple terms why the plane will take off.

Plane on a conveyor belt

How about this one: Start at the top line and try to understand why the plane moves forwards. Work your way down until you get to the bottom line, hopefully still understanding why the plane moves forwards.

I don't like Tiff - always seems like a cock to me.

Jenson seems a bit cock-like on TV, but he's reasonable face-to-face, albeit a bit of a skinny cunt. I was tempted to just give him a kicking, purely so I could say I'd given an F1 driver a kicking. Seemed a bit rude to do that though, so I didn't.

I kinda gently body-checked Tiff.. I think for the same reason. I felt silly afterwards though lol.

I only thought he was a cock cos he was "invading" on our territory pretending to be all hot shot race driver and promptly did shit.

I strangely did not have the same problem when Penny Mallory did the same thing a few years after. But then I did get to spray her with cheap champers...

Er... there is no such thing as reality. This is a hypothetical situation.

The question is perfectly designed to confuse and give different answers depending upon absolute interpretation.

In one interpretation, whatever the speed the wheels will achieve is equally matched by the conveyor belt.

The conveyor belt is designed to match the speed of the plane exactly but moves in the opposite direction. It matches the speed of the plane irrespective of the speed of the wheels. The plane never moves forwards. The rotational speed of the wheels is therefore a red herring.

Isn't this fun?

BTW. Nobody has answered my How long is a piece of string? question yet either.

Anthony Reid is a cock to the biggest degree. He was one of the "instructors" on a track day I did and was all like "i know everything about racing i am faster and better than all of you". I felt like asking him why he wasn't very good at overtaking and kept having to ram people off to get past.

The wheels don't have a speed unless they move - in this interpretation the wheels never move, they only rotate. Not the same thing at all, and not what the question says either.

The question is perfectly clear. There is no mention of the speed of the wheels - it refers only to the speed of the plane. You're just confusing the issue by suggesting the question isn't clear when it is.

Edit: I was referring to Sin.

Argghhhhhh, can't you see that that is not logical!?!?!?!

@ STB

No I understand it, this is just what lots of people seem to think, as Sin posted, and many others.

I like Tiff on the telly. But that new blonde prick on Fifth Gear is the most nauseating c*** I think I have ever seen on TV. Makes Noel Edmonds look like a good bloke.

By the way for those interested an old mate of mine called Doug owns the official TopGear clothing franchise here:

http://www.officialproducts.co.uk/

Also links from the TopGear website. He keeps offering me free gear, but I don't think an "I am the Stig" T-shirt is gonna fit too well with my Volvo.

obviously at twice the speed of the conveyor belt. what speed do you imagine ball bearings are rotating at when a skateboard wheel is rolling? twice the speed but opposite. neither have anything to do with the plane actually moving forward.

I've had 3 experiences with Tiffany Dell.

The first was at GM's private test facility at Millbrook. I'd just found a four-leafed clover (honest) near the first hairpin when he stuffed a Nissan into the bank. Radioed in the blah-blah, and as he wasn't hurt, while we were awaiting recovery and medics (for the regular check-over) I presented him with the four-leafed clover. To which he told me to, "Fuck off!".

The second time was when he was in the BTCC and dumped the car in the kitty-litter. He got out all boiling with adrenaline and rage and had a go at one of my boys. My turn to tell him to, "Fuck off!" and frog-marched him to the gap with a firm grip on his arm.

Third time I was the unlucky fool in being Chief I/O for a disasterous event called "Top Gear Live" at Silverstone. That time I had all the authority to give him a right good bollocking and threaten to take away his Racing License for 30 days... which was fun.

Sin. When in a hole stop digging.

Be a man.

Just say "OK, fair enough, I was being a bit thick" and don't try and blame DW about the wording of the question. Blaming DW for your being a bit daft is not manly.

planes only need the amount of space necessary to gain enough speed to take off. do you imagine that water-planes work with "friction" on water to get into the air? no- they just need the right amount of room to accelerate, depending on the strength of the engines how fast/slow that happens.

Now, imagine a race, with 20 odd cars, all on a conveyor belt designed to match the speed of the leading car.

With a big pit at the back of the conveyor belt.

That woudl make some interesting racing, I wonder if Tiff would be up for that (in a slow car)

Agreed, and agreed. The plane does not move.

Agreed. The wheels are a red herring.

There is no logic in hypothetical.

do you realize how retarded that sounds? a plane cannot be moving forward and standing still at the same time. it can be moving forward and have some conveyor belt underneath also moving backwards at 200mph- the free spinning wheels then render the backwards conveyor speed redundant.

@ Sin

According to you, as you've agreed with me, the only time the treadmill is at rest is when the plane is at rest. It can't stop the plane if it isn't moving, can it? Actually it can't stop it anyway unless the brakes are on but we'll ignore that for the moment.

I'm not digging.

I'm saying it won't take off in this hypothetical situation. Because I deem that the plane never produces forward momentum through air, and my hypothetical conveyor belt generates no airflow. Therefore, there is no airflow to produce lift.

Is that man enough for you?

Also, I am 99.99999% certain that this isn't DW's riddle, but he got it from somewhere else. So I'm not actually blaming anybody. I think it is a wonderful riddle.

Fuck.