MythBusters

[smack Down]

Originally posted by: Venix

Originally posted by: smack Down

Originally posted by: Venix

Here in the real world, maintaining a constant speed requires the car to overcome friction, wind resistance, and other forces. A car moving at a constant 50 MPH requires a constant output of power and is not coasting.

If the car accelerates, so does the treadmill. That's the entire premise of the problem.

Premise of which problem. There is two ways to look at the problem one where the treadmill accelerates to match the wheel speed and another where it accelerates to match the air speed. Matching of air speed can be simplified to the treadmill going at a constant speed. That is the case I was talking about.

...what? Matching the air speed means that if the plane accelerates, the treadmill accelerates at exactly the same rate. If the plane is moving at 50 MPH, so is the treadmill. If the plane accelerates to 100 MPH, so does the treadmill.

Matching the wheel speed makes absolutely no sense, since an increase in wheel speed leads to an increase in treadmill speed, which increases the wheel speed, which increases the treadmill speed...

And your point is?

 

[Venix]

Originally posted by: smack Down

Originally posted by: Venix
...what? Matching the air speed means that if the plane accelerates, the treadmill accelerates at exactly the same rate. If the plane is moving at 50 MPH, so is the treadmill. If the plane accelerates to 100 MPH, so does the treadmill.

Matching the wheel speed makes absolutely no sense, since an increase in wheel speed leads to an increase in treadmill speed, which increases the wheel speed, which increases the treadmill speed...

And your point is?

That in the first situation, a plane will move forward but a car won't. Duh.

 

[NanoStuff]

Originally posted by: Venix

Originally posted by: NanoStuff

Originally posted by: sao123
Me thinks you have no understanding of the problem itself.

Very few people, physicists included, have an understanding of the problem. Even straightdope got it wrong, as credible as the site generally is.

Straightdope's explanation has been completely consistent with all experimental results. Apparently nature itself doesn't even understand the problem.

Fact, but what they don't realize is it would be just as consistent with a car, minus the taking off of course. Straight Dope attributes the plane's ability to defeat the conveyor to it's propeller, which is wrong.

 

[randay]

apparently the no trolling rule is kind of just there for decoration.

 

[Throckmorton]

Originally posted by: NanoStuff

Originally posted by: Venix

Originally posted by: NanoStuff

Originally posted by: sao123
Me thinks you have no understanding of the problem itself.

Very few people, physicists included, have an understanding of the problem. Even straightdope got it wrong, as credible as the site generally is.

Straightdope's explanation has been completely consistent with all experimental results. Apparently nature itself doesn't even understand the problem.

Fact, but what they don't realize is it would be just as consistent with a car, minus the taking off of course. Straight Dope attributes the plane's ability to defeat the conveyor to it's propeller, which is wrong.

ARGH!! No, straightdope is not wrong. Please think about their analogy and maybe you'll understand why the plane takes off.

A thought experiment commonly cited in discussions of this question is to imagine you're standing on a health-club treadmill in rollerblades while holding a rope attached to the wall in front of you. The treadmill starts; simultaneously you begin to haul in the rope. Although you'll have to overcome some initial friction tugging you backward, in short order you'll be able to pull yourself forward easily.

And also, if for some reason you think that the treadmill moves backward at the same speed the wheels are turning forward, you also think that it's possible for X=2X

http://www.straightdope.com/columns/060203.html

 

[Throckmorton]

Originally posted by: smack Down

Originally posted by: Venix

Originally posted by: smack Down

Originally posted by: Venix

Here in the real world, maintaining a constant speed requires the car to overcome friction, wind resistance, and other forces. A car moving at a constant 50 MPH requires a constant output of power and is not coasting.

If the car accelerates, so does the treadmill. That's the entire premise of the problem.

Premise of which problem. There is two ways to look at the problem one where the treadmill accelerates to match the wheel speed and another where it accelerates to match the air speed. Matching of air speed can be simplified to the treadmill going at a constant speed. That is the case I was talking about.

...what? Matching the air speed means that if the plane accelerates, the treadmill accelerates at exactly the same rate. If the plane is moving at 50 MPH, so is the treadmill. If the plane accelerates to 100 MPH, so does the treadmill.

Matching the wheel speed makes absolutely no sense, since an increase in wheel speed leads to an increase in treadmill speed, which increases the wheel speed, which increases the treadmill speed...

And your point is?

His point is the treadmill would have to accelerate to infinite speed in 0 time.

 

[DrPizza]

nanostuff, smackdown
1st of all, welcome to the list that I've been compiling of all the people who are having difficulty comprehending English and/or Physics.

First, note that the original problem states "The plane moves in one direction, while the conveyor moves in the opposite direction." If the plane is not moving, how is the plane moving? Is that so difficult for you to grasp?

If the treadmill were to match the wheel speed, well, think about it for a second. If that were the case, it would be impossible for the wheel to ever start moving. What would make it spin forward in the first place? The *only* way to start the wheel spinning is for the plane to start moving forward. (Well, or the treadmill to suddenly move in reverse, much like pulling a tablecloth out from under a plate.)

Now, think this through for a moment: imagine that I tied a rope to the prop of a plane and started pulling it forward. While I was doing this, you were free to turn on the treadmill below the plane to whatever speed you wanted. It would have virtually no affect on the speed at which I was pulling it forward.

More: while drag force is proportional to the velocity (or proportional to the velocity squared as many models show it), frictional forces are not. Imagine having a friend of yours wearing rollerblades on a 100 yard long treadmill. You have a 110 yard rope and are pulling your friend forward at 1 or 2 mph. Someone turns on the treadmill to 5mph in reverse - you won't notice much difference. They crank the speed of the treadmill up to 100mph in reverse - again, you won't notice much of a difference from when it was moving 5mph in reverse. You'll still be just as capable of pulling your friend forward with the rope, even if the treadmill is spinning 200mph in reverse. (well, that is unless the bearings overheat)

In the case of a plane, instead of a rope pulling it forward, it's the propeller that pulls it forward. Straightdope got it right.

Oh, and about those bearings overheating:
1. Friction (well, at least static and kinetic friction) is equal to a constant times the weight. It doesn't depend on surface area, it doesn't depend on speed. Now, work is force times distance. As those bearings are spinning faster, they're traveling a greater distance inside the race. Thus, with a constant amount of frictional force, and twice the distance, you're generating twice the amount of heat. Of course, rollerblades are generally operated at speeds up to 30mph. Doubling that isn't going to cause them to fail (unless you get your rollerblades at Walmart.) Ditto the plane wheels - they're built to operate at a fairly high speed. Doubling that speed should easily be within a range that won't cause them to fail.

 

[NanoStuff]

When I said very few people I wasn't kidding, you've re-assured me enough

Now I shall vanish never to return. Debating this one is like walking on acid.

*Poof*

 

[DrPizza]

Nanostuff - I went through a bunch of your old posts on this subject. I can see where you've made a mistake. (Unless, that is, you've been trolling for nearly 2 years on this topic.)

You claim that the correct interpretation is that the treadmill matches the wheel's speed. When the problem is stated correctly, that's not a possible interpretation. When the problem is stated incorrectly, that interpretation is logically inconsistent, i.e. it leads to a paradox.

Answer the following questions and maybe it'll finally dawn on you where you've been wrong for nearly 2 years now. I haven't given up on my belief that you're not simply trolling these forums and that you actually are simply lacking in your physics ability.
Begin:

1. Imagine your plane on a treadmill. Or, it can be on ice. Or any surface that you want. The only thing that matters is that I tie a solid rope to the tail of the plane preventing it from moving forward even one billionth of an inch. The pilot tries to take off, but the plane won't go forward. The pilot cranks up the propeller full speed, but the plane refuses to budge.

questions for you to answer:
A) Do the tires spin around in place? (Possibly with much smoke from the friction) Or, do the tires just sit there, not spinning?

If you answer that they're spinning in place, then you're pretty clueless about what makes a plane move forward.
If you answer that they're not spinning, then can you explain how they will ever spin if the plane doesn't move forward? Actually, don't bother explaining - you can't. The plane must move forward for the wheels to begin spinning in the first place, which is a contradiction to your claim that it doesn't move forward. You're clinging to a belief in an impossible situation; your incorrect interpretation isn't valid.

 

[DrPizza]

Originally posted by: NanoStuff
When I said very few people I wasn't kidding, you've re-assured me enough

Now I shall vanish never to return. Debating this one is like walking on acid.

*Poof*

Actually, very few people seem to be remaining who claim the plane won't move. And, I don't know of *any* physicists who claim the plane would remain stationary or that your interpretation is even possible.

edit: and I also don't know anyone who posts as many smiley faces as you.

 

[BigJ]

Originally posted by: DrPizza
edit: and I also don't know anyone who posts as many smiley faces as you.

Head over to Video and see some posts by apoppin. Sweet Jeebus.

 

[smack Down]

Originally posted by: Venix

Originally posted by: smack Down

Originally posted by: Venix
...what? Matching the air speed means that if the plane accelerates, the treadmill accelerates at exactly the same rate. If the plane is moving at 50 MPH, so is the treadmill. If the plane accelerates to 100 MPH, so does the treadmill.

Matching the wheel speed makes absolutely no sense, since an increase in wheel speed leads to an increase in treadmill speed, which increases the wheel speed, which increases the treadmill speed...

And your point is?

That in the first situation, a plane will move forward but a car won't. Duh.

Huh, if the cars air speed is 50 mph then the car has to be moving forward. Just like the plane it will see a wheel speed of 100 mph.

 

[smack Down]

Originally posted by: DrPizza
nanostuff, smackdown
1st of all, welcome to the list that I've been compiling of all the people who are having difficulty comprehending English and/or Physics.

First, note that the original problem states "The plane moves in one direction, while the conveyor moves in the opposite direction." If the plane is not moving, how is the plane moving? Is that so difficult for you to grasp?

The plane moves just fine relative to the treadmill

If the treadmill were to match the wheel speed, well, think about it for a second. If that were the case, it would be impossible for the wheel to ever start moving. What would make it spin forward in the first place? The *only* way to start the wheel spinning is for the plane to start moving forward. (Well, or the treadmill to suddenly move in reverse, much like pulling a tablecloth out from under a plate.)

They start moving at the same instant it is called a control system. I'm not going to prove it, so you will have to take my word for it or a few college class. But it is possible to design a controlled system that has zero lag.

Now, think this through for a moment: imagine that I tied a rope to the prop of a plane and started pulling it forward. While I was doing this, you were free to turn on the treadmill below the plane to whatever speed you wanted. It would have virtually no affect on the speed at which I was pulling it forward.

Right the speed doesn't glad to see you read at least half of my post. I'm not just setting the treadmill at a speed and leaving it. The treadmill is accelerating as long as power is applied in the plane.

when I make the treadmill go from 50 mph to 100 mph for the plane to stay in one spot requires energy to increase the wheels speed from 50 mph to 100 mph. Then once you have provided the energy to increase the wheels speed to 100 mph I increase the treadmill to 200 mph requiring even more energy from you.

More: while drag force is proportional to the velocity (or proportional to the velocity squared as many models show it), frictional forces are not. Imagine having a friend of yours wearing rollerblades on a 100 yard long treadmill. You have a 110 yard rope and are pulling your friend forward at 1 or 2 mph. Someone turns on the treadmill to 5mph in reverse - you won't notice much difference. They crank the speed of the treadmill up to 100mph in reverse - again, you won't notice much of a difference from when it was moving 5mph in reverse. You'll still be just as capable of pulling your friend forward with the rope, even if the treadmill is spinning 200mph in reverse. (well, that is unless the bearings overheat)

In the case of a plane, instead of a rope pulling it forward, it's the propeller that pulls it forward. Straightdope got it right.

Oh, and about those bearings overheating:
1. Friction (well, at least static and kinetic friction) is equal to a constant times the weight. It doesn't depend on surface area, it doesn't depend on speed. Now, work is force times distance. As those bearings are spinning faster, they're traveling a greater distance inside the race. Thus, with a constant amount of frictional force, and twice the distance, you're generating twice the amount of heat. Of course, rollerblades are generally operated at speeds up to 30mph. Doubling that isn't going to cause them to fail (unless you get your rollerblades at Walmart.) Ditto the plane wheels - they're built to operate at a fairly high speed. Doubling that speed should easily be within a range that won't cause them to fail.

That great but why do I care. I have been and will continue considering the ideal case which means there is no friction (besides between the road and tire).

 

[Cold Steel]

Originally posted by: smack Down
In this thread I didn't even talk about if the plane can take off or not all I did was prove the plane behaves identical to a car.

You did no such thing because you can't. A car does NOT behave the same as a plane.

Originally posted by: smack Down
A plane, car, or any other object that can propel themselves forward on a run way can do so on a run way going backwards at any speed assuming ideal conditions. Speed is meaningless without a reference and that is what we would have in that case.

A car with the maximum possible speed of 100mph will be held stationary relative to ground when the conveyor reaches 100mph.

A plane with the maximum possible speed of 100mph will take off regardless of how fast the conveyor is going. You fail again, troll.

 

[BlackTigers]

My god, he's still going on and on.

 

[smack Down]

Originally posted by: Cold Steel

Originally posted by: smack Down
In this thread I didn't even talk about if the plane can take off or not all I did was prove the plane behaves identical to a car.

You did no such thing because you can't. A car does NOT behave the same as a plane.

Originally posted by: smack Down
A plane, car, or any other object that can propel themselves forward on a run way can do so on a run way going backwards at any speed assuming ideal conditions. Speed is meaningless without a reference and that is what we would have in that case.

A car with the maximum possible speed of 100mph will be held stationary relative to ground when the conveyor reaches 100mph.

A plane with the maximum possible speed of 100mph will take off regardless of how fast the conveyor is going. You fail again, troll.

If the planes maximum wheel speed is 100 mph and the treadmill is going faster then that then their is no way the plane can take off.

 

[Cold Steel]

Who said anything about wheel speed?? I said maximum speed.

But now that you mention it, what would cause the plane's wheel speed to max out at 100mph? The only thing maxing out a plane's wheel speed would be the condition of the bearings, grease, tires, etc. The conveyor running at 100mph certainly hasn't maxed out the plane's wheel speed.

 

[smack Down]

Originally posted by: Cold Steel
Who said anything about wheel speed?? I said maximum speed.

But now that you mention it, what would cause the plane's wheel speed to max out at 100mph? The only thing maxing out a plane's wheel speed would be the condition of the bearings, grease, tires, etc. The conveyor running at 100mph certainly hasn't maxed out the plane's wheel speed.

I don't know you are the one saying their is a max wheel speed. Or at least that is what I assumed you where talking about when you said the car would be stationary relative to the ground. Because of course all other speeds would be zero. Or where you changing your definition of speed to make the plane and car different.

 

[gsellis]

Originally posted by: BlackTigers91
My god, he's still going on and on.

The IQ will go up considerable in ATOT if they banned him.

 

[PurdueRy]

Originally posted by: smack Down

Originally posted by: Cold Steel
Who said anything about wheel speed?? I said maximum speed.

But now that you mention it, what would cause the plane's wheel speed to max out at 100mph? The only thing maxing out a plane's wheel speed would be the condition of the bearings, grease, tires, etc. The conveyor running at 100mph certainly hasn't maxed out the plane's wheel speed.

I don't know you are the one saying their is a max wheel speed. Or at least that is what I assumed you where talking about when you said the car would be stationary relative to the ground. Because of course all other speeds would be zero. Or where you changing your definition of speed to make the plane and car different.

In a car in drive without slipping wheel speed = ground speed

In an airplane Wheel speed != ground or airspeed

Hence why we ignore wheel speed for this problem.

 

[JRich]

People are still arguing? MB made it perfectly clear. THE PLANE TAKES OFF!

/thread

 

[Capt Caveman]

Originally posted by: smack Down

Originally posted by: Cold Steel

Originally posted by: smack Down
In this thread I didn't even talk about if the plane can take off or not all I did was prove the plane behaves identical to a car.

You did no such thing because you can't. A car does NOT behave the same as a plane.

Originally posted by: smack Down
A plane, car, or any other object that can propel themselves forward on a run way can do so on a run way going backwards at any speed assuming ideal conditions. Speed is meaningless without a reference and that is what we would have in that case.

A car with the maximum possible speed of 100mph will be held stationary relative to ground when the conveyor reaches 100mph.

A plane with the maximum possible speed of 100mph will take off regardless of how fast the conveyor is going. You fail again, troll.

If the planes maximum wheel speed is 100 mph and the treadmill is going faster then that then their is no way the plane can take off.

Yes, it will. The planes wheels will just skid along the treadmill, going forward and take off.

 

[smack Down]

Originally posted by: PurdueRy

Originally posted by: smack Down

Originally posted by: Cold Steel
Who said anything about wheel speed?? I said maximum speed.

But now that you mention it, what would cause the plane's wheel speed to max out at 100mph? The only thing maxing out a plane's wheel speed would be the condition of the bearings, grease, tires, etc. The conveyor running at 100mph certainly hasn't maxed out the plane's wheel speed.

I don't know you are the one saying their is a max wheel speed. Or at least that is what I assumed you where talking about when you said the car would be stationary relative to the ground. Because of course all other speeds would be zero. Or where you changing your definition of speed to make the plane and car different.

In a car in drive without slipping wheel speed = ground speed

In an airplane Wheel speed != ground or airspeed

Hence why we ignore wheel speed for this problem.

Huh, in a plane on the ground, wheel speed = ground speed or airspeed unless the ground or air is moving. That goes for a car to.

 

[Capt Caveman]

Originally posted by: smack Down

Originally posted by: PurdueRy

Originally posted by: smack Down

Originally posted by: Cold Steel
Who said anything about wheel speed?? I said maximum speed.

But now that you mention it, what would cause the plane's wheel speed to max out at 100mph? The only thing maxing out a plane's wheel speed would be the condition of the bearings, grease, tires, etc. The conveyor running at 100mph certainly hasn't maxed out the plane's wheel speed.

I don't know you are the one saying their is a max wheel speed. Or at least that is what I assumed you where talking about when you said the car would be stationary relative to the ground. Because of course all other speeds would be zero. Or where you changing your definition of speed to make the plane and car different.

In a car in drive without slipping wheel speed = ground speed

In an airplane Wheel speed != ground or airspeed

Hence why we ignore wheel speed for this problem.

Huh, in a plane on the ground, wheel speed = ground speed or airspeed unless the ground or air is moving. That goes for a car to.

Wrong.

 

[smack Down]

Originally posted by: Capt Caveman

Originally posted by: smack Down

Originally posted by: PurdueRy

Originally posted by: smack Down

Originally posted by: Cold Steel
Who said anything about wheel speed?? I said maximum speed.

But now that you mention it, what would cause the plane's wheel speed to max out at 100mph? The only thing maxing out a plane's wheel speed would be the condition of the bearings, grease, tires, etc. The conveyor running at 100mph certainly hasn't maxed out the plane's wheel speed.

I don't know you are the one saying their is a max wheel speed. Or at least that is what I assumed you where talking about when you said the car would be stationary relative to the ground. Because of course all other speeds would be zero. Or where you changing your definition of speed to make the plane and car different.

In a car in drive without slipping wheel speed = ground speed

In an airplane Wheel speed != ground or airspeed

Hence why we ignore wheel speed for this problem.

Huh, in a plane on the ground, wheel speed = ground speed or airspeed unless the ground or air is moving. That goes for a car to.

Wrong.

We are talking about the case where wheels do slip.