Physics question.. can you figure it out?

[87LtdT]

The questions states "...the plane moves in one direction..." and then states" the conveyer "...tracks the plane's speed..." and then "...tracks the plane's speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction)."

I think the question is flawed...there is no plane speed. The way I interpret the question is as soon as the engine thrust begins to move the plane the conveyer senses the movement and begins moving in the opposite direction and therefore only the wheels of the plane rotate, but the plane itself does not move with respect to the runway. As thrust causes the plane to attempt to accelerate the conveyer speed increases keeping the plane motionless with respect to the runway...but the wheels are rotating faster and faster.

No wing movement means no lift and no lift means no takeoff...however the wheels are rotating in place.

The better question, is because there is no drag to overcome (no aircraft movement), the only resistance to infinite wheel rotation rate is the tire and bearing friction...so how fast will the conveyor have to move to keep up with the ever increasing wheel speed in order to keep the plane from moving forward??

Bob

 

[wiked87gn]

i agree with Bob it wont take off..

 

[justabuickv6]

It will not and can not take off. The engine is to provide thrust. Not air flow over the wings. Thrust moves the plane forward giving it ground speed. With this, and enough air flowing over and under the wing, lift will be created.

 

[83hurstguy]

I agree with 87LtdT above... I had the same idea before reading his post.

The plane will not move forward due to the conveyor belt matching the planes forward thrust. Without forward motion, there can be no lift on the airplanes wings, and hence, it cannot fly. Using the term "matching the airplanes speed" is sort of misleading.

This conveyor belt is not like a "dyno" that just freewheels in response to a wheels force acting on it. It is like mobilizing the entire frame of reference on the ground and moving it so that it counteracts the plane's forward motion, giving it a net velocity of zero.

Without the air drag on the front of the airplane... the wheel speed will be limited to what fails first, the wheel bearings or the tires. That would have to be one fast accelerating conveyor belt.

 

[jpratt]

If you are walking on a tread mill and the tread mill is moving at the exact speed you are walking then you will go nowhere. Now if you are on the tread mill walking, nowhere of course, and decide to throw a ball the ball will fly through threw the air at the exact speed that you threw it. It does not matter how fast you are walking or runnning on that tread mill, the ball will travel at the speed you threw it.

Airplanes use thrust not wheel spin to take off. Think about a water plane. It does not have wheels that need to rotate. An ski plane lands on snow and ice, It has no wheels. The conveyor belt in this question is just there to make it harder for you to think about it. If the plane is going 100Mph down the conveyor belt, the conveyor belt will be going 100Mph the other way. The plane will be moving at 200Mph in relation to the belt but at 100Mph in relation to the rest of the world, therefore the wheels will spin very fast but the plan will still take off and fly away.

 

[Big Stu]

It'll take off, easy question.

 

[Jbenn911]

Nope, wont go..No air to create lift..No matter how much thrust is generated, it is going to stay in the same place on the runway(using the treadmill analogy, the plane being you on the treadmill, not the ball being thrown)...

 

[jpratt]

Nope, wont go..No air to create lift..No matter how much thrust is generated, it is going to stay in the same place on the runway(using the treadmill analogy, the plane being you on the treadmill, not the ball being thrown)...

Does thrust make the wheels spin? Nope thrust pushes whatever is creating the thrust. The engine create thrust therefoe the engines will be pushed by the thrust forward. The wheels will start to roll because they are being pulled forward by the engines that are creating thrust, the thrust is what is pushing everything. Wheels do nothing but hold the weight up. The plane will fly. :smile:

 

[insane6]

No it won't fly, I'm going with it needs airflow to create lift

 

[Dan Buick Man]

This question was asked on a physics forum back in July and is still being debated. http://forum.physorg.com/index.php?showtopic=2417&st=0

I think the plane will fly, an engine thrust is against air, not the ground under it.

 

[Jbenn911]

Here's a 35 year veteran pilot/engineers take on it:
"The plane will not fly. The planes motion must be forward relative to the air in
front of the wing to create the lift needed. Sitting stationary; as you would be with the conveyor moving in the opposite direction, same speed; with all the thrust in the world will create ZERO lift as the aircraft must be moving forward" He likened it to having the plane up on jackstands and the engine at WOT, plenty of power but with out the ability to move forward to create the lift, nothing happens, as the thrust created by the engines is used to propel the plane forward to create lift.

 

[turbot2496]

It'll fly. The planes not driven by the wheels nor can be held back by them without brakes. They will just freewheel backwards while the plane moves forward and picks up speed.

 

[insane6]

But it cannot move forward because the conveyer picks up speed to match

 

[jpratt]

The conveyor does not affect the air around it, thus the engines will push the plane through the air.

 

[jpratt]

http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B
Watch the skate board. As the paper is pulled out from underneath it, it still moves at the same speed, thus flight.

I did not create this, I found it on the physics thread

 

[PCS74]

From the logic side... The aircraft can have no velocity if it is not in motion. The wheels can have a rotational velocity and the runway can have a linear velocity, but the aircrafts velocity relative to a stationary reference point will be zero. We are obviously not measuring velocity relative to a moving reference point, and we are obviously not measuring angular velocity of the wheels. In the problem statement we are measuring the aircrafts velocity, so it is in motion.




Chris S

 

[PCS74]

http://videos.streetfire.net/player.aspx?fileid=35E964D9-38DB-4EFD-BE8D-D6BA1A43A06B
Watch the skate board. As the paper is pulled out from underneath it, it still moves at the same speed, thus flight.

I did not create this, I found it on the physics thread

I bet some peoples heads exploded when they saw that video.

 

[85HOT-T]

All that is happening here is the runway is passing the plane twice the speed that the plane is going. It is impossible for the runway to slow down the plane, unless the planes brakes are on.

The plane will be able to take off just as easy and as quick if the runway was not moving.

 

[Jbenn911]

Yep, mine popped ! I'd like to hear what CTX-SLPR would have to say about this one, I think he's some kind of physicist with the Air Force

 

[TurboJunky]

This is SIMPLE!

I can't believe you guys are getting tripped up on this one...IT'S SIMPLE!

Given - Airplanes fly due to lift generated by airflow over the wings thus in our scenario air must pass over the wings for flight.

Question - Will the aircraft move forward with thrust generated by jet engines?
Answer - Absolutely

Question - Does the conveyor prevent the aircraft from moving forward?
Answer - Absolutely NOT

Explanation - Since jets are propelled by THRUST from a jet engine it's movement is solely derived from AIR, not the runway. Since the forward momentum of the aircraft is INDEPENDENT of the runway or conveyor it MUST move forward creating airflow over the wings and lifting from the ground. If the vehicle were a car in your scenario it would not move since a car is dependent upon thrust by rotating tires. Jets are not. Aircraft struts/tires ONLY support the weight of the aircraft and have nothing to do with providing forward movement thus nullifying any movement by a conveyor runway.

I reject the 35 yr pilot's example because it's flawed. He likened it to the aircraft being on "jack stands". Jack stands would act as an anchor to the ground whereas the scenario presented here has the aircraft on rolling wheels = NO ANCHOR.

A famous physicist once said "for every action there must be an equal and opposite reaction". Again, the jets are pushing against the air forcing the aircraft forward. The movement of the conveyor is irrelevant.

What exactly is preventing the jet from moving forward again?....nothing.