Okay heres another scenario.

You are on a 50 foot long treadmill.

You put are on a skateboard that uses similar technology in its wheels as an airplane, and you strap the board to your feet so you cant fall off of it.

You strap a jetpack to your back and turn it up full force.

Even if the the treadmill is spinning as fast in the opposite direction as the jetpack is pushing you in the other, you are still going to move forward because all the treadmill is doing is making your wheels spin. The wheels aren't pushing you forward, the jetpack is.

The wheels are solidly connected to the skateboard, right?

The treadmill is solidly connected to the ground?

The velocity of the bottom section of the wheel in relation to the skateboard is equal to the negative velocity of the treadmill surface in relation to the ground?

If the answer to all of these is yes, the skateboard will have no velocity relative to the ground.

0 + X + (-X) + 0 = 0

Always.

It's the same scenario. For you to move forward, your wheels have to roll - and when your wheels roll, the treadmill moves under it and you go nowhere.

If you lift your feet off the ground, you'll go forward - but then that's the wreck scenario I mentioned earlier (wheels come off).

The resistance to overcome for the plane comes from the air on the wings.

The resistance to overcome for the car comes from friction on the tires.

Big difference....

but the treadmill doesn't have the capability of affecting the plane's momentum or movement

so when it tries to do that all it can do is make the wheel spin even faster while the plane takes off

well explained MCJ

No. There's never any movement so there's never any air pressure on the wings. That's why planes have to "take off" - that is, drive forward to a certain speed before going airborne.

Say you are at an airport with those people mover sidewalks. You are wearing roller skates. The sidewalk is going north to south at 5 ft/sec relative to the ground. There is also a tow rope going south to north, also at 5 ft/sec relative to the ground. You enter at the south end (going with the tow rope and against the side walk). You grab the tow rope and step on the side walk.

What is your velocity relative to the ground?

What is the velocity of the wheels?

Assume that your body is inelastic.

Read that carefully. That's a motor-driven conveyor belt. It's not a free-spinning treadmill that matches the plane's wheel energy.*



Dan • Jan 30 2008 • 10:51PM
No Greg, the experiment is right, you are wrong. You confuse speed with force. The treadmill can move with an opposing speed to the plane, but it cannot apply an equal and opposing force, because wheel bearings transmit only a very small rolling friction. The question nowhere states that the treadmill counters the plane's thrust or keeps it stationary. It only that it moves in the opposite direction at the same speed.


*This may not be the original post that started this, but it's the assumption I layed out in #126.

This is not the same scenario. The wheels and the side walk are not moving at the same speed.

Who said that?

Even this would work if the treadmill was attached to a mass equal to the plane that would have to be propelled along with it - which would satisfy the first sentence.

The planes wheels dont thrust it forward, so the treadmill cannot counteract its thrust.

Its this simple.

A cars wheels thrust it foward, so the treadmill can counteract its thrust.

What is there not to get?

My car doesn't have rockets strapped to the roof. If it did, a treadmill wouldn't do a damn thing to keep it in place, even if the speedometer were reading "0".

The conveyor belt is totally irrelevant compared to the dynamics of flight.

The plane's jets push air in the opposite direction. The air that is pushed in the opposite direction of the plane is not affected by the ground. The only thing the tires do is hold the plane off the ground until a sufficient amount of air is pushed behind the plan to gain lift from the airflow over the shape of the wings.

A care pushes the ground in the opposite direction. Completely totally different.