The plane exerts virtually no force against the ground. The wheels spin freely (other than friction coming from the rotation around the axle). The vast majority of the force that the plane expends is against the air. Thus, the speed of the ground in relation to the plane has virtually no bearing on anything. It's only the speed of the air passing around it. And, in fact, if there was a headwind that matched the plane's takeoff speed, it wouldn't move forward, but it would still leave the ground. (Theoretically. I'm sure that getting the wind to blow in that consistent a direction at that consistent a speed is virtually impossible. Maybe in a wind tunnel.)

Gotcha. Makes sense.

I too could not understand why they even did this test. The outcome just seemed so obvious. However I'm sure my wife, (who is very intelligent, but not when it comes to physics) would have been confused.

In this episode they killed bugs with radiation, in San Francisco. I wonder how may protesters they had at their door the next day.

That's the root of the puzzle! No matter how hard you try, you can't make *that* happen!

As soon as the plane throttles up, it's gonna move forward. Doesn't matter how hard you pull backwards on that conveyor belt, the plane goes forward at more or less the same speed that it would whether the conveyor belt was there or not.

You can only keep the plane from taking off in two ways: 1. Applying its brakes. 2. *Not* throttling up. But those would keep the plane from taking off whether there was a conveyor belt or not.

Here's another visualization of it that Vixy mentioned to me as we were watching the episode last night: What if we took flight out of the puzzle completely? What if the plane were just taxiing? The puzzle ends up being exactly the same, with the same result. If you throttle the plane up to the place where it would normally taxi at, say, 10mph, and you pull back on that conveyor belt at 10, 20, 30, or even 50mph, the plane will still move forward at 10mph.

Not so rare, actually. This is why light planes are tied down at airports. Airplanes take off and land into the wind, of course, so that less space is required. WWII aircraft carriers sailed full speed into the wind for aircraft launches for this very reason.

What's more, it's perfectly possible to have negative ground speed when flying into a headwind- all you need is a mountain pass to funnel the air up to high velocity (or higher than the flying speed of the aircraft).
I'm reminded of a story told by Richard Bach about flying a biplane across the country. I can't remember which book it was in- perhaps Nothing By Chance. He had to cross such a pass at low levels where the wind speed was less (wind speed is slower close to ground due to friction with the ground). He scared the heck out of a few drivers who happened to be on the road, but it was his only way through. At higher altitudes the wind drove him backwards.

The bicycle pushes on the treadmill. The airplane pushes on the air. There is NO power being transmitted to the wheels.

Hm. CanuckInOR's post to which this was a reply has vanished.

Damn, Bitt... you're too fast on the reply -- you got me while I was in the middle of deleting my post because I realized my mistake, which, sadly, was exactly the same mistake described in the straight dope, only with a bicycle, instead of a car.

Iwas just about to post this as well.

If you put the bike on the treadmill, tied a role to the wall in front of the bike, then turn the treadmill on, it would not matter how fast the treadmill was turning you could still pull the bike forward. The force you use to pull it forward would only be slightly greater than if the treadmill was not running. The only extra force required is the fiction force from your wheel bearings.

The airplane example is the same thing only the force it uses is pushing against air, not pulling a rope.

Having just been one of the confused (though I got it the first time round), may I suggest that it has less to do with lack of knowledge about physics, and more to do with just not fully understanding how planes are powered, and, consequently, trying to relate it to a powered vehicle that they do know about? Quite simply, if you forget (or never knew in the first place) that a plane's engine doesn't power the wheels to make it move, you're going to be stumped.

Heh, yeah... what confused me was Vixy's suggestion to take flight out of the question.

And, FWIW, I'll not delete posts ever again...

Thank heavens it's Friday... now I just have to hide long enough 'til I can go home from work. :-p

I suppose the people who make this mistake assume that the plane puts power to its wheels until the moment it needs to get airborne? In other words, vroom drive down the runway really fast and then LEAP into the air at which point you start flapping your wings?

I think that's exactly what's happening. Have you ever set up a computer for grandparents who, to underline something, use the backspace key and delete what they just created? That's how you do it on a typewriter.

Yeah, why not? If you don't know how the mechanics work, then it's a reasonable assumption to make. A wheeled vehicle moves forward by having the engine turn the wheels. A plane is a wheeled vehicle with wings. Therefore...


I'd flap my wings as I vroomed down the runway -- like a duck taking off from a lake!

Okay so it is the opposite direction, but a few years back my dad landed a Britten Norman Islander (9 seater, twin engined, high wing, go anywhere plane) slightly in reverse onto the apron, immediately adjacent to the gate. Of course this was in Orkney, which is considerably windier than the rest of the UK.

He said it was dead easy. Wind speed of 90 knots meant it was inevitable.

Wow that's a pretty big plane to be landing backwards. Must have been a sphincter factor of 9.5 on that one.

You're also going to be stumped if you forget (or never knew) that the friction forces on the wheels don't increase as the wheels' speed increases -- which is a whole lot less intuitive, IMO.

The dialogue "Does the plane take off?" -- "No" -- "Yes it does" is, in a way, a very roundabout version of the dialogue "Is there a factor for relative speed in the equations of friction?" -- "Yes" -- "No there isn't".

Peter

I've done that kind of landing with an R/C model. The biggest problem was dealing with the air turbulence. The plane wanted to dart up and down.

90 knots? That's a category 2 hurricane! I bet plenty of airport buildings in the UK wouldn't have survived that...

Peter

LOL Wow- landing backwards! That's one for the log books.

That Britten-Norman Islander sure does remind me of an Aero Commander. Very similar in appearance and size. I'm sure I would have mistaken an Islander for an Aero Commander, not ever having heard of Britten-Norman before.

Interesting that John Britten, one of the aircraft's designers, has a namesake known especially to motorcycle enthusiasts.

That's only relevant if you'd think that the wheels couldn't turn twice as fast as normal takeoff speed. Seems unlikely that an airplane would be engineered with tolerances that low, though I could be wrong.

Only if you are too lazy to think on to what happens to keep it in the air once the wheels leave the ground !

Fairy dust and happy thoughts?

I've managed this on several occasions with a ducted fan powered glider with about a 2 metre wingspan. Our flying field is fairly high up and has quite laminar wind flow patterns if the wind is coming from the right direction and is fast enough. It's perfectly possible to have the thing land completely vertically from several hundred feet up, while maintaining an ordinary level flight angle. It looks almost impossible, the plane simple sinks gently down until it touches the ground.

It's a very good example of the difference between ground speed and air speed. The plane is, of course, doing some 25-30 miles per hour relative to the air, so is flying completely normally, but the air is doing exactly the same speed in the opposite direction relative to the ground so the plane just hangs there in much the same way bricks don't

pca

Nope. Lift demons.

I can offer personal testimonial that this works

Go to the top of a windy hill.
Strap yourself to a hang glider.
Have some (experienced) people hold ropes tied to the glider.
Angle the glider into the wind.
Rise vertically into the air.
Practise all the banking, climbing and descent controls in safety just a few feet off the ground.

Great fun!

I gave almost the same example in December 2005 in this very thread.

Instead of a bicycle I proposed a toy car on a treadmill held in your hand.

The first comment I made in that post sums it all up though. In a plane crash, where do you bury the survivors?

You don't.

Does anyone know the Mythbusters episode name that featured this? I want to search for it on my PVR.

I have an extremely intelligent friend that simply cannot wrap his head around this problem. He is just having a hard time separating airspeed from ground speed. For some reason he's having a mental block about the air above the belt not moving WITH the belt.

Another thing I thought was very interesting was that the pilot they hired to test the myth believed the plane would sit there "like a brick". This guy is a stunt pilot. Huh?! How could he not intuitively know this?

That's exactly what the Mythbusters did.

Episode name? I'm pretty sure it's titled "Airplane on a Conveyor Belt."

I've just been through this with Mrs lbt and she thought it wouldn't take off. Anyway, after explaining that she should have known better and she was, therefore, a plank; I am now out of bed and in the study typing <sigh>.

OK - so that got me thinking... given there was some ambiguity about the wording of the question : what should the 'correct' question have been?

I'm not even mentioning this bloody riddle to Erin. We're supposed to be getting married in April and I don't want it getting in the way.

On Nikoncafe.com the same subject has now gone to 70 pages - all within the past two weeks or so. Heh.