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cdcox 06-04-2009 10:55 PM

prhom --

Does the fact that the planes wheels are spinning fast somehow counteract the thrust generated by the jet engine?

That force will cause the plane to accelerate relative to the ground unless there is some counteracting force. Short of the wheel bearing seizing that force does not exist. Unless you want to suspend Newton's 2nd law.

prhom 06-04-2009 10:55 PM

Quote:

Originally Posted by cdcox (Post 5818780)
Now you are depending on the bearing burning up and seizing in order to keep the jet from taking off. I think that violates the spirit of the brain teaser.

No, because as the extra force pushing the plane forward is translated into faster wheel rotation the treadmill matches it.

orange 06-04-2009 10:57 PM

Quote:

Originally Posted by prhom (Post 5818783)
I'm going to bed after this because frankly, I'm exhausted now. Since the treadmill has an unlimited capacity to accelerate it could theoretically continue accelerating until the friction in the wheel bearings was greater than the acceleration force of the plane. Again, in the real world everything would burn up in this ridiculous scenario pretty fast.

The wheels/treadmill will not accelerate indefinitely. In the real world, both have some friction which will produce great drag at higher rotational velocity. The wheels will reach a maximum speed - matched by the treadmill - and the system will run in equilibrium as long as the jet fuel lasts.

cdcox 06-04-2009 10:57 PM

Quote:

Originally Posted by orange (Post 5818784)
The rocket cars. Already mentioned.

How is a rocket car different from a match box car with a hand pushing it? Do you have a video of a rocket car spinning on a tread mill?

cdcox 06-04-2009 11:00 PM

Quote:

Originally Posted by orange (Post 5818782)
speed of jet wheels (depends on) - speed of treadmill (equals wheel speed) =
speed of treadmill (equals wheel speed) + speed of jet (determined by force balance) - speed of treadmill (equals wheel speed)

speed of jet wheels (depends on) - speed of treadmill (equals wheel speed) =
speed of treadmill (equals wheel speed) + speed of jet (determined by force balance) - speed of treadmill (equals wheel speed)


0 = speed of jet (determined by force balance)

The force balance on my jet makes it go 5 mph at a particular instant. What are you going to set your treadmill at?

Joe Seahawk 06-04-2009 11:01 PM

Wheels do not matter when the jet is moved by the thrust of the jets.. the wheels and the treadmill are nearly irrelevant. This seems very simple to me.. Is something wrong with me? :D It's almost like saying a jet couldn't take off if it had skis instead of wheels and the substrate was ice.

cdcox 06-04-2009 11:01 PM

Quote:

Originally Posted by prhom (Post 5818783)
I'm going to bed after this because frankly, I'm exhausted now. Since the treadmill has an unlimited capacity to accelerate it could theoretically continue accelerating until the friction in the wheel bearings was greater than the acceleration force of the plane. Again, in the real world everything would burn up in this ridiculous scenario pretty fast.

BTW, Oranges treadmill doesn't have a motor. It free wheels.

orange 06-04-2009 11:01 PM

Quote:

Originally Posted by cdcox (Post 5818790)
How is a rocket car different from a match box car with a hand pushing it? Do you have a video of a rocket car spinning on a tread mill?

The hand pushing the car has to move forward. The rocket engine expels mass out the back. It can produce energy to move the car while standing in place.

prhom 06-04-2009 11:02 PM

Quote:

Originally Posted by cdcox (Post 5818786)
prhom --

Does the fact that the planes wheels are spinning fast somehow counteract the thrust generated by the jet engine?

That force will cause the plane to accelerate relative to the ground unless there is some counteracting force. Short of the wheel bearing seizing that force does not exist. Unless you want to suspend Newton's 2nd law.

See #437, because the treadmill can accelerate its not a static problem. It's a dynamics problem now, the extra force gets counteracted by some magical force propelling the treadmill to MATCH the speed of the wheel. It's tricky because the wheel and treadmill are by definition going the same speed as long as they are in contact with one another. So you can substitute matched speed with matched force. As soon as the plane starts to "speed up" relative to the treadmill the treadmill will respond with added speed from some magical engine that provides that forces. It's that external magical force that makes this a stupid question.

orange 06-04-2009 11:02 PM

Quote:

Originally Posted by cdcox (Post 5818800)
BTW, Oranges treadmill doesn't have a motor. It free wheels.

Less parts to burn up. It gets all the energy it needs to perfectly match the wheels - from the wheels (ultimately from the jet engines).

prhom 06-04-2009 11:04 PM

Quote:

Originally Posted by Joe Seahawk (Post 5818798)
Wheels do not matter when the jet is moved by the thrust of the jets.. the wheels and the treadmill are nearly irrelevant. This seems very simple to me.. Is something wrong with me? :D

Unfortunately, yes, because the treadmill can accelerate. If the treadmill is not powered or runs at a fixed speed then the plane might have a chance depending on how fast the treadmill can go and how much power the plane has.

orange 06-04-2009 11:04 PM

Quote:

Originally Posted by Joe Seahawk (Post 5818798)
Wheels do not matter when the jet is moved by the thrust of the jets.. the wheels and the treadmill are nearly irrelevant. This seems very simple to me.. Is something wrong with me? :D It's almost like saying a jet couldn't take off if it had skis instead of wheels and the substrate was ice.

If you think it's a very simple question - yes there's something wrong with you. Try googling it. Baby Lee mentioned one site that has 300 PAGES of comments.

And there are many more sites.

And the ice in your example is incorrect. It's not moving relative to the rest of the world. The ground under the plane in the problem is. Mud is a better example.

prhom 06-04-2009 11:06 PM

Quote:

Originally Posted by orange (Post 5818803)
Less parts to burn up. It gets all the energy it needs to perfectly match the wheels - from the wheels (ultimately from the jet engines).

That's where you are incorrect orange. The treadmill must have an external force propelling the treads to counteract the plane engine thrust.

cdcox 06-04-2009 11:06 PM

Quote:

Originally Posted by Joe Seahawk (Post 5818798)
Wheels do not matter when the jet is moved by the thrust of the jets.. the wheels and the treadmill are nearly irrelevant. This seems very simple to me.. Is something wrong with me? :D

No. There is something wrong with me for keep trying to come up with different explanations in an attempt to teach.

orange 06-04-2009 11:09 PM

Quote:

Originally Posted by prhom (Post 5818809)
That's where you are incorrect orange. The treadmill must have an external force propelling the treads to counteract the plane engine thrust.

The plane engine thrust affects the wheels and the treadmill equally. It doesn't have to be counteracted.


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