Dad,

Force of thrust of jet engine >>>>>>> Force of friction

And I'm spent.

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I've seen (with my own two eyes) a J-3 Cub fly into a strong enough headwind to go backward. Airspeed 48 mph, Groundspeed -5.

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In the 60s, one of my brothers took off in an airplane in wind that was faster than the airplane's stall speed.

Another brother and my sister's husband each held a wing. When he was ready to take off, he set the throttle, they let go of the wings, and the airplane rolled forward a few feet and took off.

I think that was the time when his destination was another town about 60 miles away directly into the wind and he nearly had to turn around before covering 60 miles, return to the airport he took off from, and refuel.

idiots, all of you

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The only way I can explain it is the treadmill/rollerblades/rope example. No matter how fast the treadmill is spinning, you can still pull yourself forward with the rope or handrails.

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If the conveyor under the rolleblades matches the speed of the wheels, you will succeed in pulling yourself over and falling on your face, then you will be able to drag your injured self off.

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How does a jogger stay on a treadmill? Same principle. If the treadmill matches the joggers speed no matter how fast the jogger runs, the jogger will never run off the front of the treadmill.

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Kramer,

The difference is the jogger is applying force to the belt to move forward. The speed of the belt would keep the jogger from moving forward.

Do you really think a plane takes off from a normal runway because of the wheels?

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Force of thrust of jet engine >>>>>>> Force of friction

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Not according to how the problem is set up. With an actual treadmill and wheels, yes, but the problem doesn't state that. If these wheels have friction, no matter how minimal, the treadmill can go fast enough to hold the plane in place.

Well, if the plane is held "in place", then the treadmill should be operating at 0mph. That's the setup of the problem.


Thanks for playing!
WHAT?

The wheels are irrelevant. They are free turning and are only there to provide a

If you put the airplane on the belt with no engines running and turned the belt on (disregarding the matching speed thing) the plane would go backward (the wheels don't turn, the plane just rides the belt backward, because the "ground" is moving). Likewise, if you fire the engines up on a regular runway, the airplane moves down the runway (wheels turn, ground stays still).

So if you allowed the wheels to turn AND the ground to move (note: the wheels only turn due to the force of the thrust of the engines. the wheels don't move the plane, the engines do), the plane wouldn't move.

The ice analogy is false because in that example, the ground is not exerting a force on the plane.

Put the kid on the skateboard on a threadmill with rails on the side. He pulls the rails forward, but that force is matched and countered perfectly by the treadmill. The wheels on the skateboard are irrelevant. The wheels don't exert any force in this equation so they cannot have an outcome on the equation.

It all comes down to the force of the engines vs. the force of the conveyour belt. If the question is taken literally, they are balanced and no movement will result.

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Dad,

Force of thrust of jet engine >>>>>>> Force of friction

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In real life, yes, but in this imaginary situation, if there is no slippage between the conveyor and the wheels, then that friction force is never overcome.

I choose to believe the phrase in the original question "..matches the speed of the wheels" to mean that there is never any slippage between the wheels and the conveyor belt.

The entire force of the engine's thrust is magically transferred to the spinning conveyor belt causing the plane to remain motionless and the pilot to say WTF!?!?

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the force of the conveyour belt

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What force is the conveyor belt applying to the plane?

Where does "slippage" come into play. When a plane takes off on a stationary runway there is no slippage, there's rolling.

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What force is the conveyor belt applying to the plane?

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Friction.

Wheels turn. It's what they do.

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What force is the conveyor belt applying to the plane?

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I read the scenario as if the conveyor belt is applying a force to the wheels equal to that force that the wheels are applying to the conveyor belt. Absent losses due to friction, etc... that would be identical to the force of the engine's thrust. (assuming the plane is not weightless)

the wheels have no power, but have to be able to move foward in order to generate some sort of air movement across the wings, no? with the conveyor belt moving at the same speed, the wheels would turn, but never move foward and there would be no airflow so the plane would not take off. please tell me where my logic is wrong

"TRACKS THE SPEED OF THE PLANE".

As if a cop was sitting beside the runway with his radar gun. He sees the plane moving at 2mph forward, the conveyor goes 2mph backward.

This only increases the rotational speed of the wheels. It does NOT make the plane stop moving.

If the problem were to specify matching the speed of the wheels, then we'd need to pin down exactly which speed, since there are two that people keep confusing.

The first speed is relative to the ground. That's also the speed of the airplane.

The second "speed" is the tangential speed along the contact point. This depends on the wheel's position. At the contact point to the ground, its speed is 0. Think about a tank track staying put as it rolls by. The speed of the wheel at the top of its path is equal to twice the first mentioned speed.

But the problem only specifies the speed of the plane. People thinking about the speed of the wheels need to stop.

Well, this A&M graduate, Corps Member, and Gulf War Veteran also happens to have majored at A&M in...Aerospace Engineering.

Sorry boys, but I think this guy knows his stuff.

Conveyor belt + plane = ConAir.

Put... the bunny... back... in the... box

i know they dont exert any force, but dont they have to be able to move fowards in order for the plane to move?

big champ, like I said in my previous post, it all depends on how you look at the question, now that I've gone back and looked at it again, I can see both sides. the people that say the plane will not fly do so because they assume the plane is not moving. no movement = no windspeed = no lift. this is 100% correct. I also see you guys side of it now by looking at the question and assuming the plane IS moving forward. the conveyor just makes the wheels spin twice as fast as if on stationary runway. as the rate at which the wheels spin matters as much as what I ate for breakfast, if the plane is moving forward, that gives airspeed, that gives lift.

Kramer, for the record, which side are you exactly on? Doesn't take off?

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Where does "slippage" come into play. When a plane takes off on a stationary runway there is no slippage, there's rolling.

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Exactly, it is that rolling that allows the plane to move across the ground more easily. If there were no wheels, there would be the greater friction of scraping along the ground that would be overcome.

In our scenario, however, the wheels roll in place rather than across the ground.

There's another way to say it! If the conveyor belt results in the wheels just rolling in place rather than with any forward motion, then the plane does not take off. If the conveyor belt "matches the speed" of the wheels, that is the same as saying the wheels only roll in place. If the thrust is then enough to overcome that and allow the wheels to slide along the conveyor belt, then the plane takes off. That is what I mean by "slippage".

Kramer....I would take out the bit about being in the corps....you were trying to get respect weren't you?

so the plane is moving?

the plane is moving

ok if the plane is moving, then yes it will take off. i was under the impression that the wheels were just spinning in place because the conveyor belt was matching the force of the propellor.

Goose has credentials ... he went to Top Gun.

http://www.texags.com/main/forum.reply.asp?forum_id=12&topic_id=574344

Whoa there guinea... read the initial post. The speed of the PLANE is monitored, and the belt adjusted accordingly.

If you could move the conveyor belt fast enough that the friction in the wheel bearings was great enough to counteract the thrust of the engines, the resulting heat in the wheel bearings would very quickly liquify them and the surrounding metal. The airplane would quickly crash onto the belt.

But that's not possible. If nothing else, the tires themselves would catch fire and/or disintegrate long before that point.

So that interpretation of the questio is just plain silly.

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As if a cop was sitting beside the runway with his radar gun. He sees the plane moving at 2mph forward, the conveyor goes 2mph backward.

This only increases the rotational speed of the wheels. It does NOT make the plane stop moving.

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If the plane is going 2 mph forward (in relation to the surrounding area) and the conveyor belt is going 2 mph backwards, then the wheels of the plane would have to sliding on the conveyor rather than just turning.

of course it will take off.


i need to learn how to read

The question itself is silly. If we can have the treadmill match the plane's speed with ZERO reaction time then you can also say that the wheels are indestructable.