Physics question.. can you figure it out?

[myclone]

I retract my previous incorrect theory . I was 100% focused what the air flow across the wings to generate lift vs forward motion would do and forgot about the thrust from the props/turbines pushing against the stationary atmosphere is what creates theforward movement/lift.

I dont feel so bad though as EVERY coworker that read the question got it wrong too

 

[John Larkin]

The plane cannot take off. It cannot create lift if it does not move forward assuming the conveyer truly negates the forward motion of the wheels 1:1. It is no different than sitting ready for takeoff with the brakes locked and engines straining; we've all experienced this as passengers. We did not suddenly lift into the air from thrust alone.

 

[TType84]

The plane cannot take off. It cannot create lift if it does not move forward assuming the conveyer truly negates the forward motion of the wheels 1:1. It is no different than sitting ready for takeoff with the brakes locked and engines straining; we've all experienced this as passengers. We did not suddenly lift into the air from thrust alone.

the conveyor matches the wheelspeed.

not the thrust.

the drag on the wheels will never match the forward thrust from the engines.



you guys did great on this.. people actually thought it through without getting pissed off closing their minds.

:biggrin:

 

[jpratt]

Nevermind.

 

[TurboJunky]

The plane cannot take off. It cannot create lift if it does not move forward assuming the conveyer truly negates the forward motion of the wheels 1:1. It is no different than sitting ready for takeoff with the brakes locked and engines straining; we've all experienced this as passengers. We did not suddenly lift into the air from thrust alone.

So you're saying the aircraft's engines cannot move the plane forward? If so, why not?

Assumimg our airplane is an F-15 Eagle which produces 29,000 lbs of thrust in each engine. What's pushing against it with 58,000 lbs of force to keep it from moving forward?

"For every action there's an equal and opposite reaction."
- Sir Isaac Newton

 

[John Larkin]

You read my opinion. I don't have to prove anything. Why do you have to challenge me personally? Geez....

 

[jpratt]

You read my opinion. I don't have to prove anything. Why do you have to challenge me personally? Geez....

The whole purpose of this thread was to debat, challenge each other. I think it has opened a few eyes to a new way of thinking.

 

[TurboJunky]

You read my opinion. I don't have to prove anything. Why do you have to challenge me personally? Geez....

Who me?? I'm not personally challenging you whatsoever. I thought this was a very thoughtful and intellectual debate. I apologize if you think I was somehow insulting.

I was only asking you where the opposing energy was coming from with 58,000 lbs of force to keep the plane stationary is all. If this question was offensive I'm sorry.

For those who don't believe the plane will move have you seen the video posted of the skateboard testing this very debate? If you haven't, the speed of the skateboard WAS NOT affected by the "runway" moving in the opposite direction. The skateboard's wheels just spun faster.

 

[DavzGN]

It can't be answered!!!

There isn't enough information to answer. We don't know how long the conveyor runway is. If the conveyor isn't long enough, the plane can't take off.


David
87GN

 

[EightSecV6]

If the plane relied on its wheels as the driving force to obtain air speed, it could never take off (like a race car on the conveyer, it wouldnt move UNLESS it was a jet car) but it does not, it relies on thrust. It will take off no matter how fast the conveyer turns.

 

[mydesigner]

The skateboard video he was barely moving the paper. The equal and opposite reaction is the runway moving. Thrust of the plane pushes the plane forward and turns the wheels, the runway then moves the opposite direction. Suppose for example you have a water plane facing upstream. The stream is flowing at 100 mph. The plane would have to have an airspeed of 100 just to stay in the same spot, and would not take off cause it is not actually moving. There is no airflow over the wings to cause the lift necessary. Thrust of the plane has to match the river before it can even start moving.

 

[83hurstguy]

Thrust of the plane pushes the plane forward and turns the wheels, the runway then moves the opposite direction. Suppose for example you have a water plane facing upstream. The stream is flowing at 100 mph. The plane would have to have an airspeed of 100 just to stay in the same spot, and would not take off cause it is not actually moving. There is no airflow over the wings to cause the lift necessary. Thrust of the plane has to match the river before it can even start moving.

First, the planes engines exert no direct torque to turn the wheels and force the plane forward. Hence, the wheels are not driving the aircraft, and there is no equal/opposite reaction. The only 3rd law force couple in the wheels is the aircrafts weight and normal force applied back from the ground.

Secondly, the water plane in a stream analogy is not valid in comparison to an aircraft on a conveyor belt on land. There are no "wheels" that allow the aircraft to move over the water without dragging on it. The wheels between the belt and aircraft's fuselage are the key to the aircraft being able to take off. The water essentially causes a drag force on the aircraft, and the plane will accelerate with a net force equal to that of engine thrust minus the drag force from the moving water. Cut the wheels off the plane on the conveyor belt, and you get the same effect, only with more heat, sparks, and fire.

 

[TurboJunky]

The skateboard video he was barely moving the paper. The equal and opposite reaction is the runway moving. Thrust of the plane pushes the plane forward and turns the wheels, the runway then moves the opposite direction. Suppose for example you have a water plane facing upstream. The stream is flowing at 100 mph. The plane would have to have an airspeed of 100 just to stay in the same spot, and would not take off cause it is not actually moving. There is no airflow over the wings to cause the lift necessary. Thrust of the plane has to match the river before it can even start moving.

Good point but I think it's flawed. When you're talking about free moving wheels on a level surface that's moving at 100 mph you don't need the HP to keep the vehicle stationary as opposed to a 100 mph river with a float plane. It's all about drag. The float plane is experiencing a lot more drag than one with tires on a hard, level surface. The one on a hard level surface requires much less HP to keep it stationary or move it forward than the one with much more drag.

Let's analyze this in a practical sense:

1. Plane throttles up and begins to move at 5 mph and increases speed.
2. Conveyor moves in opposite direction moving at 5 mph and increases speed. I see the plane continuing forward at 5 mph, increasing speed but the tires are moving backward at 10 mph and increasing speed since the plane is using thrust against the atmosphere (air) and not the moving ground.
3. This continues exponentially until the aircraft reaches V1/V2 and lifts off the ground.

 

[jpratt]

The skateboard video he was barely moving the paper. The equal and opposite reaction is the runway moving. Thrust of the plane pushes the plane forward and turns the wheels, the runway then moves the opposite direction. Suppose for example you have a water plane facing upstream. The stream is flowing at 100 mph. The plane would have to have an airspeed of 100 just to stay in the same spot, and would not take off cause it is not actually moving. There is no airflow over the wings to cause the lift necessary. Thrust of the plane has to match the river before it can even start moving.

The stream theory would work BUT only if you was talking about a propeller under the water.

 

[TurboJunky]

Here's a great explanation I found on a physics board:

Folks this is a no brainer.

The aircraft has engines that through action/reaction principles cause it to accelerate in relation to the AIR not the belt.As the aircraft starts to move the belt will move in the opposite direction, the only effect this will have is to increase the speed of rotation of the tires, the plane will continue to accelerate in relation to the air and the ground beside the moving belt no matter how fast the belt moves. This may have detrimental effects on the tires but will not detrimentally affect the AIRSPEED of the plane. In fact, air entrained by laminar boundary layers on the belt may actually enhance the airflow to the wings, shortening the takeoff run. Depends on how long the landing gear legs are. In the 60's a plane was built with a hovercraft type skirt instead of regular gear, this plane wouldn't even notice the increasing speed of the belt.

 

[TurboJunky]

I want to change the original scenario a bit to favor the "anti-flight" crowd.

Let's say the conveyor belt moves backward at double the speed of the aircraft's expected speed based on engine RPM. What then?

PHYSICS FORUM DISCUSSION - The plane will fly.

 

[mydesigner]

We need to test this anyone have a RC plane and a treadmill? I went to mythbusters website and their forum has a current thread of like 36 pages. I hope that they test this.

 

[Buick From Hell]

FOR EVERY ACTION THERE'S AN EQUAL AND OPPOSITE REACTION...

In the case of the conveyor & plane, think of it this way:

The planes wheels are on the conveyor, and 10 strong men, NOT on the conveyor, are holding ropes attached to the plane. The conveyor starts moving, spinning the tires, but the men hold the plane in place ....the conveyor speeds up...Soon the conveyor is spinning the tires at 200 mph, yet the men continue to hold the plane in place...

Now, the pilot hits the throttle on the jets...Will the men STILL be able to hold the plane in place?

And one more log for the fire: Rockets can push objects in the complete ABSENCE of air...equal and opposite reaction.

 

[SPOOLFOOL2]

I agree that if the plane can not move forward in relationship to the wind and produce air speed, that it cant fly. I also read the question as if it says that the conveyor belt matches the planes wheel speed as to keep it stationary. As in matching the forward thrust of the plane against the runway through the friction of the wheels and bearings. So, can this be practicly done? No. But, neather can a tred mill runway. Insted of thinking of a powerfull jet, think of a Piper Cub with a low hp engine and almost flat tires. As it generates enough thrust too move it forward 1 mph, the the runway pushes back with enough force (say 4 mph) to hold the plane nutral. the wheels are now turning at 4 mph. The plane increases power to try to obtain 5mph but, the runway speeds up to 20 mph (and so on) keeping the plane nutral to the the earth and the wind. NOW, if while all this was happening, a wind that was higher than the stall speed of the plane blew down the runway and across the wings of the plane then,YES... It would fly .


Fun thread.

Good night all.

 

[turbo buicks]

the plane flies. given the hovercraft example its easy to see the conveyor/wheel speed has no bearing on actual forward movement in relation to a fixed ground point. also take the bike example. get on a bike and start a conveyor. more u pedal the faster it goes. now strap a JATO on the back and stop pedaling and turn up the belt speed to match the thrust output of the JATO. ur gonna fly off the conveyor with the wheels spinnin 10000000000000 mph. forward movement in relation to the air = lift. you can do the same thing at home by holding a bicycle from a moving car. tell me how much force it takes to push it forward at 20, 30, 40, etc mph while holding on to it out the window. same force. the only way to stop the planes getting fwd airspeed would be to place a counteracting barrier/force against the front of the plane. also take a plane flying normally @ 700 mph. extend landing gear. place moving conveyor under plane. does it just lose lift b/c the wheels start to spin fast, no. i remember i used to push a little truck on my grandmas treadmill when i was little and i remember i could turn the treadmill up hi as i wanted and it would still take the same power to move it forward or keep it steady no matter the tread speed. u could put a plane on a conveyor while sittin still and ramp the belt to 100 mph. this would SEEM to put the plane at a disadvantage but it doesnt b/c the wheels spin 100 mph and the plane goes nowhere. if a plane was dependent on wheels for THRUST it would move backward (and it wouldnt fly) so you have belt speed vs forward airspeed 100 = 0. now add 100 lbs force to back of plane. wheels spin 200 mph and plane moves +100 mph while conveyor moves 100 mph. theyre equal now but planes going 100 mph forward, GAINING LIFT! i only debated this with my dad for a few minutes and it makes sense it would fly.