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# Re: [Phys-L] Impossible vehicles.

Regarding Brian W.'s reponse:

David ripostes that a sail boat's contact with the ground is unnecessary if
the channel can be tacked up or down IN STILL AIR.I urge him to review
the requirements for faster than windspeed travel <g>

I urge Brian to review how a Galilean transformation transforms those requirements.

If the air is calm WRT the riverbank then there is a wind blowing relative to the water (i.e. in a reference frame in which the water in the main channel is at rest) in the upstream direction, and the shoreline is also moving in the upstream direction at that same airspeed. When the sailboat tacks downwind faster than that wind the boat travels in the upstream direction faster than the shoreline moves in the upstream direction, and thus makes net progress upstream relative to the shoreline. When the boat sails into the wind, tacking appropriately, faster than the wind it makes net progress downstream relative to the water, and thus moves downstream faster than the river's current relative to the shoreline.

Now suppose the boat doesn't tack at all and sort of goes with the flow with its sail partially unfurled. It then travels downstream relative to the shoreline at a speed somewhat slower than the current because of the resistance it gets from the calm air. From the point of view of the water in the main channel the boat moves somewhat in the upstream direction because of the upstream wind, but the *water's* resistance on the hull keeps it from acquiring the full speed of the wind. From the point of view of the boat itself the wind and the water are moving in opposite directions and forces from their flows past the hull and the sail cancel out so the boat remains stationary, as it must, in its own rest frame.

David Bowman