rationalskepticism.org

Onyx8 wrote:

spork wrote:Four things to keep in mind:
1) The prop never turns the wheels. The wheels turn the prop

So what starts the wheels turning?

Net force on the cart starts it rolling, wheels start turning.

Onyx8 wrote:

spork wrote:
2) Inertia doesn't factor into it. Even exactly at wind speed we can tow a trailer all day long.

Would you mind backing that assertion up?

You cannot use your own inertia to accelerate yourself. When the cart accelerates, all its parts accelerate as well. So the cart in not using any inertia for acceleration.

Onyx8 wrote:

spork wrote:
The blades are following a spiraling path so they will always feel relative wind even when the cart is going downwind at wind speed - remember their tangential velocity.

Actually they are traveling in a helical path, but nevertheless they are still traveling at exactly the speed of the wind.

Wrong, you forgot the tangential velocity. At wind speed, the blade velocity vector is the vector sum true_wind + tangential_velocity. Different speed and different direction than the air.

Onyx8 wrote:

spork wrote:4) When the cart is going the speed of the wind - it feels no wind. But that doesn't mean there is no wind. It simply means there is no relative wind. There's still wind moving over the ground, and that's precisely the energy this vehicle exploits. It leaves a path of slower moving air in its wake - having taken some of its energy.

How does it extract energy from air that is not moving relative to itself?

Push it back to slow it down.