Mythbusters - airplane on a conveyor belt

I think the answer to our discussion lies in where the thrust is being supplied. (Feel free to make sexual jokes...) If you put a car on a conveyer belt, the car wouldn't move forward because its thrust is supplied through its wheels, which are in direct contact with the conveyer belt. Wheel speed matches conveyer belt speed, and the car goes nowhere, meaning no airflow over the vehicle.

The plane's thrust is supplied completely independent of the conveyer belt, so those wheels can spin as fast as they want, while the propeller pulls the plane forward against the stable air, creating air movement over the wings and pulling the plane into the air.

If you sat yourself on a skateboard on a treadmill, you could still move forward if you pushed against the stable ground around you.

Ohio Rulz wrote:

A genius like yourself, if you took the time to think about the post, would've figured out that the plane at "rest" - with wheels spinning away - was PRIOR to the engines being turned on in my scenario.

Good attempt at condescension, though. Too bad your ignorance trumped your efforts.

Fair enough if true. Or you could explicitly say it how you want it to mean so that there is no confusion.

How does a sea plane generate lift?

The missing link wrote:

If you sat yourself on a skateboard on a treadmill, you could still move forward if you pushed against the stable ground around you.

The only problem with this is that you'd be causing the skateboard wheel speed (rotation) to exceed the treadmill's speed. That's what causes you to move forward. In the plane scenario, the treadmill keeps up - its speed always matches wheel speed. Its speed is dynamic and fluid, not "set." This is what throws most people.

THUS, I contend that the thrust of the engines, when turned on and pushing against stable air (your point), would push the plane forward with all sorts of noise and screeching. The force on plane going forward would overwhelm whatever friction is keeping the wheel and treadmill spinning at the same rate.

I suppose the only way to keep the plane grounded would be to extremeeeeeely slowly build up the thrust, allowing the wheels and treadmill to increase their rate of rotation slowly. This could go on forever, assuming the treadmill and wheels would hold up.

However, a big ole blast of the engine would get it off the ground in a shower of sparks, burnt rubber, broken wheels, and general chaos.

That clear enough for you, physics grad?

Ohio Rulz wrote:

In the plane scenario, the treadmill keeps up - its speed always matches wheel speed. Its speed is dynamic and fluid, not "set." This is what throws most people.

If that's true then answer me this:

Suppose the plane's engines are generating 1000 N of forward thrust. (I don't know anything about jet engines or Newtons, that could really be any number, it doesn't matter.)

What speed does the treadmill turn at to keep the plane from moving forward?

What force vector is cancelling out the 1000 N of thrust?

If you can answer either of those questions I will gladly accept your argument. Otherwise, I am convinced that the plane ABSOLUTELY WILL take off without excessive difficulty.

Not trying to harangue you, but think about the setup. Wheel speed can only equal treadmill speed if the plane is not moving. So you've basically said "If the plane is not moving, then the wheel speed = treadmill speed. Therefore the plane is not moving"

a qestion to all thinking that the plane will take off. once the wheels of the plane lose contact with the treamill what happens to the plane?

Does it suddenly accelerate to cruising speed?

Does it stall out and crash?

If this treadmill plan works, why doesn't the Navy use it on their aircraft carriers??

You fail to understand the whole intricacy of why how the question is worded can have a big impact on what exactly is happening.

1. The argument that the plane takes off is predicated upon the plane moving forward through the air.

2. For the plane to move forward through the air, it is necessary for the plane's wheels to be moving faster than the treadmill.

3. If the treadmill's speed matches the plane's wheels, the plane does not move forward through the air and the plane does not take off.

Show me where I am wrong.

I agree - for those who say the plane can move thru the air - how fast are the wheels going and how fast is the treadmill going while the plane is moving forward thru the air?

Physics professor wrote:

3. If the treadmill's speed matches the plane's wheels, the plane does not move forward through the air and the plane does not take off.

I'm going to be an ass and keep disputing this. You can't just say that wheel speed = treadmill speed because this can ONLY happen IF the plane is not moving forward. There has to be a reason the plane is not moving forward. There isn't enough frictional force for the wheels alone to be that reason.

Right, but assume neither the plane and the treadmill are not moving. Then the plane's engines come on and the wheels begin to move. As long as the treadmill equals the wheel speed, the plane does not move forward through the air and does not take off.

The traditional wording of the riddle DOES say that treadmill speed = wheel speed. Therefore, no plane movement and no take off.

According to their web site and TV Guide, last night's episode of Mythbusters was supposed to address the airplane on a treadmill question. They didn't and nerds everywhere are upset. According to an email from the executive producer of the show, the segment got rescheduled:

First up, for those concerned that this story has been cancelled, don't worry, planes on a conveyer belt has been filmed, is spectacular, and will be part of what us Mythbusters refer to as 'episode 97'. Currently that is due to air on January 30th.

Secondly, for those very aggrieved fans feeling "duped" into watching tonight's show, I can only apologise. I'm not sure why the listings / internet advertised that tonight's show contained POCB. I will endeavour to find out an answer but for those conspiracy theorists amongst you, I can assure you that it will have just been an honest mistake.

Not sure that's going to quench the nerdfury, but I'm glad the piece will air in January.

not again wrote:

What speed does the treadmill turn at to keep the plane from moving forward?

What force vector is cancelling out the 1000 N of thrust?

If you can answer either of those questions I will gladly accept your argument. Otherwise, I am convinced that the plane ABSOLUTELY WILL take off without excessive difficulty.

Dude, we agree. Your 1000N of thrust has no opposing force. What people THINK is that the increased rate of the spinning wheels will absorb that force. (In other words, your 1000N simply causes the wheels - and treadmill - to just spin way faster, and the plane stays still). No freakin' way.

The plane is going forward, no matter what. The "gear system," if you will, of the treadmill and wheels - created by gravity and friction - will fail immediately. Even if the treadmill were programmed to increase its rate, the contact of wheel-treadmill wouldn't be maintained. Hence the screechy, noisy take-off mess.

OK thanks I can die happy now.

Yes, good point. I see the problem in my earlier question about needing 300 MPH worth of thrust to get to 150 MPH if the treadmill is going 150 MPH in the opposite direction. In reality you'd only need 150 MPH + friction of tires on belt.

BUT ... the question, at least in the Boingboing link above, does say that the treadmill is going the same speed as the wheels. Does that mean the same speed the wheels are rotating (which to me means the plane is stationary), or does that mean the same speed the wheels are moving across the ground (which to me means it is going forward as fast as the treadmill is going in the opposite direction)?

Physics professor wrote:

1. The argument that the plane takes off is predicated upon the plane moving forward through the air.

2. For the plane to move forward through the air, it is necessary for the plane's wheels to be moving faster than the treadmill.

3. If the treadmill's speed matches the plane's wheels, the plane does not move forward through the air and the plane does not take off.

Show me where I am wrong.

Reading the discussion on the page i linked to above, I believe this is a correct answer. Points to consider:

1. In reality, the fact that there IS rolling friction between the wheels and the treadmill belt means that, lacking a force in the opposite direction, the treadmill would be able to move the plane backward. However, the frictional force to overcome is small in relation to the thrust an airplane's engines could produce (this process works the same regardless of whether the treadmill is running)

2. This means that if thrust equal to the force of the rolling resistance is applied, the airplane should be stationary while the belt and wheels move at equal speeds, but opposite directions. Any further increase in thrust should therefore lead to a positive acceleration force and the airplane should theoretically be able to take off eventually.

3. This scenario, however, as physics professor just wrote, means that the wheels will begin spinning at a faster rate than the treadmill belt, and thus may indeed violate the conditions of the problem, if it is interpreted to mean that the wheels cannot spin faster than the treadmill.

4. There also seems to be some consensus that in reality, if the treadmill were able to exactly match the speed of the wheels, both the belt and wheel speed would almost instantly accelerate to infinity, resulting in the disintegration of the wheels before the plane could take off anyway.