[felson]

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Doesn't:

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The runway moves in the opposite direction of the plane at the exact same speed as the plane's wheels.


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mean no net movement? Same speed, opposite direction.


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I would really like to see a response to this. So far all I'm hearing is, "the wheel speed doesn't matter, only the plane speed matters." Well, of course. But the conditions of the problem are impossible to satisfy unless the plane groundspeed is zero, or the wheels are slipping a ridiculous amount. IMO the problem is ill-posed and doesn't deserve all this attention.

I'm also confused why people say that the fan-on-skateboard contraption won't move because there is no net force on the system. Would you say the same thing if the sail were just 1 square centimeter in area? If not, then any explanation must take into account the size of the sail. I don't believe that has happened yet, except for some people who observed that the sail cannot capture all the moving air blown by the fan.