Airplane physics question

I'm trying to keep it simple here Art. I said ignore acceleration did I not?
Yes, I did

In reality, would there be a change in force, yes, because the wheels need to accelerate up to 10mph, the bearings would produce a slightly higher force at this speed etc. But, for the simplest case, where the change in speed is instantaneous, and the bearing friction change is negligible, there would be no change in force required to pull you.

 

Nowhere in the question is action and reaction mentioned. Read the speed of the treadmill, spin up your rollerblade wheels to the same speed, then put them on the treadmill. They won't go anywhere.

We can ignore friction. It's a very small force compare to the othe rmuch larger forces at work here.

It's about forces. Forces result in acceleration on a mass. You can't ignore it. If you agree with f=ma, then you cannot ignore it. Do you agree with f=ma?

If I understand this correctly, then it will not happen if the person is running in place and the treadmill is matching his speed and he does not exert any incremental energy. If he is expending 3 calories per minute to stay in place, he will need to spend 4 calories per minute to stay in place and pull his buddy on the rollerblades. If the rollerblades have very low friction and the buddy is very light, maybe he will need to expend 3.00004 calories. But there will be an additional expenditure of energy.

For the person on the rollerblades to get up on the treadmill, he has to be accelerated. There it is. It cannot be avoided. For him to be accelerated, a force has to act on him. This force comes from you and is transmitted through the rope you have attached to yourself and him. It's all about forces. And forces result in acceleration and deceleration on masses.

 

Yes, the bearings decrease the forces required to move objects. That's why we have them! But it does not change the underlying physics principles. You need to exert a force to move an object at rest. Or to stop an object in motion. This change in state is called acceleration. You cannot get away from it.

 

Is there a (ahem) landing strip nearby

 

OK, ignoring the slight increase in friction, which I agree is negligible anyways, the additional energy is spent to accelerate the wheels from 5mph to 10mph so that they can continue on at 5mph in relation to the sidewalk, adn at 10mph in relation to the treadmill. The wheels have mass. Mass requires force to be accelerated. If it's a very small mass, then you need to supply a very small amount of force. But you do have to provide it. The force has to come from you, since you are the only source of thrust. Put a hookscale between you and the wagon, and it will register this additional force you have to come up with, in order to maintain 5mph. It might be very small, but it's there. To see it, simply increase the weight of the wagon wheels.

Or go ask any physics professor.

Do you disagree?

 

Art forget acceleration. It is confusing you. The concept of this problem can be treated as steady state. Do you know what that means, or why it can be treated as such?

 

1. Yes, but as they do, the conveyor rolls under them at the same speed. So they don't move forward. Neither does the plane.
2. The belt is not delivering ANY energy to the wheels.

Yes, they are efficient and smooth like a frictionless bearing, or like our plane's wheels on the conveyor belt. In no wind, with fast enough water flow, the plane is going to sit there, it's engine going faster and faster. On days with no wind, why do you think their takeoff speeds vary with the current?

 

I know what steady state means. You can have steady state with constant force (causing acceleration), if there is an equal and opposite force (cause equal and opposite acceleration) present. This is actually how you maintain constant speed.

 

If you want to completely ignore acceleration, then you cannot introduce any new forces such as thrust into the equation. Want to do that? Fine.

The wheels and conveyor belt are rotating in opposite direction at the same speed. Therefore, though the wheels are rotating, they are not moving forward along the conveyor belt. Since they are not moving forward, neither is the plane that is attached to them. Simple as that. It sits there happily, it's wheels churning away on top of the conveyor churning away in the oppsite direction. What's the problem?

Surely you agree that in order to change position in relation to the conveyor belt, it has to move forward? Moving forward from rest will require a force to provide acceleration that will change the object's horizontal speed to above zero. Agree?

 

Bullplop. Why can't thrust be in a steady state problem. Assume thrust is constant.

 

Shiggins,

I think Art is having you on...... hook line and sinker.

Rick

 

It can be, as long as you realize that the steady is due to opposing forces that cancel. Agreed? Remember, thrust is a force. Forces cause acceleration. For ti to be steady state, it would have to be constantly accelerating, and you would have to account for that. I don't think you want this. I think you want steady speed. So, you could simply have an opposing and equal force, canceling the acceleration. Now you have steady state and steady speed. The thrust and force and acceleration are still there, they are just being cancelled. Agreed?

 

haha, I mentioned that I thought he was doing that before, and he said he wasn't. I think you're right though.

 

I was assuming the wagon was on skis, on an icy sidewalk with a frictionless conveyor.

 

Yes, a natural blonde one.

 

Art,
Lets assume a perfect wheel/axle system. The wheels rotate around the axel with no friction, therefore there is no force transmitted to the axel, landing gear, etc. from the wheels rotating. Right?
Now put this system (and plane) on the conveyor belt. Turn on the conveyor belt, with the belt moving in the direction of nose to tail. Will the plane move backwards? No, because there is no linear force being transmitted through the wheels. The plane is at a standstill with the wheels smoothly rotating in unison with the conveyor belt.
Now take this same system, with the conveyor belt off, and power up the engine. Will the plane move forward? Yes, because the wheels are providing no resistance to the forward thrust of the engine.
Now the sceanrio is set, the conveyor belt does not transmit any opposing force to the forward motion of the plane (as supported by the first example) and the plane moves forward up to take off speed (as supported by the second example) and ultimately takes off.
The motion of the plane forward and the conveyor belt are independent of each other because the wheels separate the opposing linear motions.

 

Iteration # 157......

Art gets it. He has to.

 

4 more posts and we will be at post 747. Maybe then I will get it. I sure don't yet.

Here is the simple question that I am asking. How can you be rolling on a surface that perfectly matches your speed in the other direction (meaning you are standing still in terms of horizontal movement) and yet be moving along the surface (which clearly implies it is not matching your horizontal speed in the opposite direction).

Answer that one simple question. You can't.

 

Simple, just because the surface is perfectly matching your speed doesn't mean that there is a means for the forces to transmit themselves to your body.
FREEROLLING

 

Freerolling neither implies faster, nor does it imply slower. All it implies is that there is no engine directly connected to it, driving it.

It can be freerolling and still it will need to be faster freerolling than the speed of the treadmill in the opposite direction. The question states that it is not faster at any time. Its speed is the same. Not faster. Not slower. Now tell me how it freerolls up the treadmill? For it to do that, it HAS to go faster than the belt going in the opposite direction.

 

Let us look at the energy balance of the airplane as the engine fires up.
Without the conveyor belt moving, at any given time, we have

energy input e(t) from the engine must be equal to kinetic energy
of the plane plus rotational energy of the tires (neglecting friction).

e(t) = 1/2 * m * v(t)^s + 1/2 T * omega^2

where v is plane velocity, m is plane mass, T is moment of inertia
of wheels/tires and omega is angular velocity of wheel(s)

without conveyor belt, the energy of the engine goes mostly into
linear acceleration of the plane since the moment of inertia of the
wheels is so small.

However, with conveyor belt running and exactly matching tire speed at any
given time, v(t) = 0 and all energy of the engine gets converted into rotational energy of the wheels which will be spinning at incredibly high speed
due to the lack of any energy transfer between tires and belt. therefore the plane will not take of since its velocity relative to the ground must alwas be zero.

note that the basic assumption is that the belt "knows" at any given time
what the speed of the airplane will be. so, to really create a correct model,
you would have to accelerate the belt speed in exactly the same way the plane would
accelerate (without taking air resistance into account).

 

No, it implies that as little force as possible is transmitted to the vehicles body. If you have no resistance to forward motion, you have forward motion. The freerolling wheels see to it that there is little to no resistance to forward motion, certainly not anywhere near enough to prevent a plane from taking off. That's what they're designed to do. Who cares how fast they're spinning, it doesn't matter.

 

Thank you thank you thank you. One more person who believes in science instead of magic.

 

You are right about very little force being transmitted back to the plane. You are wrong about movement (it's actually acceleration, but you don't like that word). That is why the wheel spins up to such high speeds. The force has to go somewhere!!!!! Since there is very little resistance, very little force is taken up in negating the resistance. Where does the rest go? It goes to the first moving object that can be moved (accelerated, but we won't mention that) and that object starts moving (accelerating). Since there is a lot of force and very little force is needed to move (accelerate) the object, it moves (accelerates) the light object to a very high speed.

That was 10th grade physics.

If there was no friction, the plane would slide down the conveyor belt without the wheels rotating. It would then take off. This is what happens to ski planes on snow. Very, very little friction. Rigid objects (skis) that don't give in. First thing to give is the small coefficient of friction. As soon as it's overcome, the plane moves forward.

Using the same skiplane, if you put skis on the end of hinged poles that bend very easily (with virtually no force), guess what happens when you fire up the engine. The thrust force will cause the poles to before the the plane starts moving. Why? Because the object that moves with the smallest force will move first, as soon as its threshold of force is attained.

That was 11th grade physics.

teak360's Kandy lady gets on the ski plane.

That's 12th grade stuff. We can't talk about that until after prom night.

 

This Art, is not science, it is some pseudo voodoo magically delicious gibber jabber. He assumes if the conveyer is matching the speed, the plane has no forward velocity. Wrong, prove it, don't assume it. You can't skip the premise of the question. The energy of the engine doesn't magically get converted to rotational motion of the wheels unless the plane is moving, there is no connection other than motion of the plane. WHOA, hold on that violates the faulty assumption he made at the beginning.

Tires cant spin unless the plane moves. Conveyer doesn't move unless the tires moves. Assuming v(t)=0 means nothing moves. Further, assuming v(t)=0, while the engines are thrusting means **** is going to break, not that the tires will magically harness the thrust and start rotating. Since we're on wheels, it won't break, it will move, forwards.