Airplane physics question

You are right. I meant a plane with floats. You know that.

Yes, agreed. I thought I needed to get into the details because people were having a hard time understand how a disconnected jet could "drive" a wheel.

They don't power a plane, but they do affect its ability to move because they facilitate (or hinder) the plane's movement on a solid surface. If the solid surface is moving backwards under a forward rotating wheel, at the same speed, the wheel is not going to change its horizontal position. Neither will anything attached to it, even a big airplane.

The plane's movement is critical for airflow. Airflow is critical for lift. Lift is critical for take off. Done deal.

 

i had to....

but irregardless is not a word... sorry it was buggin me

 

Wrong again.

It is entirely possible to accelerate on a conveyor going at a constant speed in the opposite direction. You are right so far. It is IMPOSSIBLE to move forwards on a conveyor that is accelerating backwards at the same rate that you are accelerating forwards at. Simple as that. You will just get the freerolling wheels to spin faster. But they will not move forwards. If they don't move fowards, then there is no horizontal motion. No horizontal motion, no airflow...

 

HOW? for god sakes man, how can the airplanes freerolling wheels and a conveyer belt prevent forward motion? You have yet to answer this, just answer it. How is this physically possible.

 

See, I didn't get that at all - I thought that the ENTIRE reason for stating that the treadmill counteracted the ground speed was not to suggest ground accelleration was possible. Heck, i got the impression that the treadmill was specifically for this purpose.

 

I will admit to not having read every post here but it seems that a basic point has been missed. If the wheels start to move they have a speed that the belt can match but the moment they match the plane will contiue to increase wheel speed to satisfy the equation . The intial speed may have been canceled by the belt but the new speed is just now being met . the speed of the wheel is dependent on the plane moving foward . If this is not the case then the belt is driving the wheel and the original question is no longer being asked . at some point the plane will continue to increase the wheel speed so the belt can match it and take off velocity will be reached.

 

I know you did, that's why I mentioned the huge drag, the drag that disqualifies this analogy.

It can't drive it, it can only drag it along with the plane.

Therein lies the impossibility, why even consider it with regard to a Newtonian physics experiment?

All those left standing here understand airflow is critical to lift. All but one understand that a question that creates an impossible situation cannot have a valid answer.

 

No Art, no no no no no no no.

It is possible. Forget acceleration. Lets just look at constant speeds. It's a steady state question, acceleration just complicates it.

I have a wagon, it's red, and you like it, so you get in. I pull you at 5mph. Don't worry about acceleration, it's irrelevant. We reaced 5 mph and I'm pulling you along at that speed. Suddenly, one of the block in the sidewalk turns into a conveyer belt moving backwards at 5mph. I'm scared of conveyer belts, so I walk around it on the grass, but keep pulling you along the sidewalk at exactly 5mph onto the conveyer belt. I will be able to pull you over the conveyer at 5mph with no extra force, of we neglect the slight increase in rolling friction. But, if for some reason you were hanging over the edge, staring at the outer edge of the wheel. You would think think you were going at 10mph, because the wheel would be spinning twice as fast as it was on solid ground.

You can't possibly think that as soon as you hit that conveyer I wouldn't beable to pull you across without accelerating you.

 

A lot of people thought that. But then what's the point of the question. It won't fly, game over. If it can't gain airspeed, it can't fly. The question just becomes redundant.

 

Shiggins is correct. If you pay attention to the original question as posed in this thread, then the physics don't work. However, if you understand what the "real" question is (as posted by shiggins), then shiggins physics are correct.
The plane will move forard with thrust, the wheels will rotate at twice the translational speed(due to the forces of the conveyor belt), and the plane will take off at essentially its normal take off speed and distance.
The conveyor belt is not opposing the translational direction of the plane or the wheels on the axles.(i.e. forward motion created by the thrust of the engine). The translational direction (and potential opposing forces) of the conveyor belt and the plane are independent, because the free rolling wheels are providing an efficient (i.e. low resistance) physical separation of the movement of the plane and conveyor belt. The wheels will simply roll over the top of the moving conveyor belt. The only energy the conveyor belt is delivering to the wheels is rotational, not directional. The sea plane will also take off with an opposing current because the pontoons ride on top of the water, there is very little opposing forces between them and the water, and the plane and pontoons simply slide across the top of the water very effeciently.

 

You're sort of there, but not quite. I suggest you not risk your sanity by entering this thread without reading every post. But by then you will have become insane, as have Mr Payne, Shiggins and I have.

 

LOL! Do you agree that roller blade wheels are freerolling? If so, go stand on a treadmill with them. Can you stand still? Of course you can. So now we have proven that they can rotate yet the mass attached to them (you) via the axles does not have to move.

Now all we have to do is keep you standing still while a force tries to accelerate you, while the treadmill also speeds up in the opposite direction to match your wheel speed at all times. Agreed?

OK, grab a fan and turn it on. Have a friend speed up the treadmill. You should be able to stay still despite the additional thrust of the fan.

You can also do this with a skateboard. The thing is to be very exact in your measurements.

Or, you can simply read the question that says that the treadmill matches the speed at all times. Implicit in this is that the acceleration is matched at all times. Otherwise, with different accelerations, you would end up with different speeds. The question says this does not happen. So, you can prove to yourself that it is possible, or just accept it because the question is asking you to. Now that that's out of the way, how the heck are you going to move up the treadmill if it goes faster backwards every time you go faster forwards? And how are you going to get airflow on a calm day if you can't move forwards?

No airflow=...

 

Hi Folks,

As I see it, the conveyor belt can be going backwards in relation to the plane's wheels in both the translational sense AND in the rotational sense WITHOUT breaking the constraints of the question (as originally worded in this thread).

The reason I think this is that when the wheels on the plane are physically moving forwards, [unless they are slipping, which wouldn't happen in this scenario], the rotational speed will always be, exactly equal and opposite the conveyors motion.

Say a wheel (translationally) is rolling along at 5mph (in relation to the ground), held down against the ground such that it doesn't slip against the ground. The wheel is doing 5mph in relation the ground and the rotational speed of the wheel is n revolutions per minute (value of n depends on the size of the wheel, of course). The ground, likewise, is moving at 5mph in relation to the wheel, just like houses moving in relation to a car driving down the road.

I hope you agree the speed of the wheel is equal and opposite the speed of the ground, in both the translational and rotational senses.

Now say the wheel (translationally) not going anywhere in relation to the earth, held down against a conveyor who's belt *is* moving in relation to the earth. The wheel is rotating because its tyre's face is in contact with the conveyor. This time the conveyor might be going at, say 7mph in relation to the wheel, the band of the conveyor is still 'translating' at 7mph in relation to the wheel, and the wheel is moving at 7mph in relation to the conveyor under it. Also, the rotational speed of the tyre exactly matches the rotational speed of the conveyor, only in opposite directions.

So, fast-forwarding from the question up to now, we've got a plane on the conveyor and it intends to take off. The engines can push the plane which moves the wheels forward via their centres and they turn due to the couple (pair of forces acting offset from each other) between the axle and the contact patch of the tyre. If the conveyor belt is set to move at a speed backwards, the sum of all the forces on the wheels will make the rotation of the wheels exactly equal and opposite to the conveyor's motion, whatever that conveyor speed is and whatever the translational speed of the plane is. So the question of the rotations being 'different' is just a nothing unless there is some force such as brakes introduced in to the equation (which there isn't).

If you also decide to make the translational speed of the conveyor equal and opposite to the translational speed of the wheel's centre, (or the "plane") then you just have a special case of the above - when the translational speed of the plane's wheels is 10mph forwards, the translational speed of the conveyor is 10mph backwards and the wheels just turn as if they are doing 20mph - but in relation to the conveyor belt, they ARE doing 20mph, in BOTH senses, translational and rotational.

Nowhere in the question does it say that the rotational speed of the conveyor must be equal but opposite to 'the speed the wheels would be doing if the plane were moving down a fixed runway'.

I don't agree with comparing the rotational speed of the conveyor with the ground yet comparing the rotational speed of the plane's wheels in relation to conveyor - that's not the way to compare speeds.

Please, either compare the speeds of things to a constant reference point, or compare them to each other, but don't compare one item to one ref point and another item to a different ref point and then claim that the constraints of the question are broken. Translational speeds can be compared between any two points in space (because a translational speed is just a distance that changes over time). Rotational speeds must be compared with other items that are rotating (it makes no sense to ask how the rotation of the second hand on my watch relates to the translational speed I'm driving down the road). In this case the two rotating items in question are the wheels and the conveyor belt.

Because of the above, as I see it the plane can move forward, get lift and take off even with the constraints applied in the most rigorous sense.

To shoot down the argument that says that the conveyor would zoom up to some amazing speed: to do that the plane's wheels would actually have to slip on the conveyor, because only then would they be turning faster. Again, this is based on measuring the relative speeds correctly, as described above. As there are no forces to make the wheels slip against the conveyor, this situation doesn't occur, the rotational speeds ALWAYS match and also the conveyor matches the translational speed of the wheel, and thus the most rigorous way to interpret the constraints can still be met.

Another way to shoot down the argument that says the conveyor shoots up to an amazing speed is that the question states that the conveyor EXACTLY matches the speed of the wheels - the ever-accelerating-conveyor argument relies on the plane's wheels rotating faster than the conveyor for a period of time and the conveyor reacting to that (which immediately breaks the constraint, so this can't happen). If I even try to make the argument work, I have to bring the time at which the plane's wheels are rotating faster than the conveyor down to zero. If I do this, at EXACTLY the same time ANY force disturbs the non-moving start condition, the conveyor MUST accelerate to the highest possible speed (its trying not to allow the plane to move, right?) and that speed would be infinity. Yet because its accelerating to infinity in ZERO time it must also occupy ALL of the speeds between ZERO and INFINITY in the the SAME INSTANT. This just cannot happen - the only way the constraint can be kept yet allow the conveyor belt to accelerate is IF THE PLANE MOVES ALONG THE CONVEYOR.

Best Regards,

Rich.

 

The treadmill is exactly matching the speed at all times. This is why it is not driving them. If it was slower, it would be braking them. Faster, it would be accelerating them. It's doing neither. It's exactly matching the wheels. So they are rotating, but not moving forwards or backwards. Neither is the plane that is sitting on top of them.

 

You are now a member of "The Original Question is a Conundrum Society" too.

 

If the freerolling wheel has no translational motion, it will have no rotational motion, and the conveyer will not be moving.

 

You forgot to add "which is why the original question is invalid"

 

Art:

You are in an airplane flying at an altitude of 5,280 feet.
A woman passenger says "Art, I would like to join the Mile High Club"
What do you do?

 

Dead wrong. Do you know any physics professors? Go ask them. You do need extra force to pull me once I am on the conveyor belt. Because you apply this extra force (you barely feel it because of the average human strength vs weight in this situation) the wheels accelerate to 10 mph.

Here's a better variation of the same experiment. Pull me with a string that just barely can stand the strain at 5mph. Get the wagon on the conveyor and try to maintain your speed on the ground...the string will break. It is the weakest point in the chain and will show you that there are additional forces being transmitted from you to the wagon.

You can also use a spring scale and see the "weight" that allows you to achieve the steady state on the pavement. Now, when you get on the conveyor and maintain the same speed, you will see the scale register a greater "weight" which is really you pulling on it (and through it, the wagon) with more force.

 

Is this in Nevada?

 

It's valid. The two speeds match at all times. What's the problem? Ever seen two cars pace eachother on the highway? The speeds match. What if they had a remote controlled brake servo and throttle. Could they accelerate and decelerate at the same time? Of course. Could you stand them up on their trunks and have their tires rotate against each other while you did this, taking direction of rotation into account? Of course. What if you put a belt around the wheels of one car? You could do that too. What's the problem?

 

Yes, they are freerolling. In reality, not you wouldn't stay still, bearing friction would cause you to move backwards. But, that is easily overcome with little forward force. But, here, you are assuming the treadmill initiates the motion. That is not the system we are considering. Our treadmill initially reacts to the plane/wheel motion. A freerolling wheel cannot move unless the body it's attached to moves forward. Can it roll without the body moving, sure, in a case like you describe, but that's not what we're discussing.

What wheel speed is it matching? You're making it too complicated. Forget acceleration, forget it, it's completely irrelevant in this question. It is a steady state question.

There is a simple experiment you could do. Two treadmills, rollerblades, two people, and some rope. Face the treadmills towards each other. Have one person put on rollerblades, and the other stand in his shoes. Have both people stand on the opposing treadmills and attach the rope to each other. Turn on your conveyers to the same speed, keeping in mind they're going in opposite directions to each other. Now, at the same time, both of you get on. I guaratee, the person in his shoes will pull the person on the roller blades up the treadmill. Despite the fact that the treadmill under the rollerbladers feet is matching the translational velocity of the rollerblade wheel.

 

Yup. 100% correct. No acceleration from rest means no motion. No motion means no airflow... teak360, you know how that goes!

 

Shiggins said to ignore the slight increase in friction, which is proper in a Newtonian Physics thought experiment.

 

Yes and she is from Las Vegas, here name is Kandy Kane.