PhilGoetz comments on The Correct Contrarian Cluster - Less Wrong

38 Post author: Eliezer_Yudkowsky 21 December 2009 10:01PM

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Comment author: Nick_Tarleton 23 December 2009 05:41:00AM 3 points [-]

(Completely OT, of course.)

When the vehicle is moving as fast as the wind, in order to go faster, the energy output from the propeller must be more than the energy input through the wheels. The energy output of the propeller comes entirely from the energy input through the wheels, so this is impossible.

Looking at it in the road's reference frame, the propeller decelerates the wind — even if the vehicle is already moving at wind speed — and takes kinetic energy from it.

Comment author: PhilGoetz 23 December 2009 05:55:41AM 0 points [-]

The idea is that the propeller is providing thrust, not taking energy from the wind. It's rotating in the opposite direction from what you're suggesting.

Comment author: Nick_Tarleton 23 December 2009 06:07:16AM *  3 points [-]

The propeller does both. If the vehicle is moving at the same speed as the wind, then in the vehicle frame, the wind is being accelerated backwards (hence momentum is conserved), so in the road frame, the wind is being decelerated and donating energy to the vehicle.

Comment author: PhilGoetz 23 December 2009 06:24:42AM 0 points [-]

The movement of the wind backwards is coupled to the movement of the vehicle forward; but that's the effect of the energy, not the source of the energy.

Comment author: wedrifid 23 December 2009 06:43:59AM 2 points [-]

If your intuitions don't think it will work then two options available are building the device or doing the actual math.

The movement of the wind backwards is coupled to the movement of the vehicle forward; but that's the effect of the energy, not the source of the energy.

My intuition tells me that the source of the energy is the wind and some of that energy is removed from the wind and ends up on the cart.

The idea is that the propeller is providing thrust, not taking energy from the wind.

Think push not twist. The energy taken from the wind is not in the form of increased rotation of the blade. Rather, it is being pushed along like a sail. It just happens to put some of the energy back into increased rotational energy of the blade by means of gears connected to the ground.

Comment author: PhilGoetz 23 December 2009 07:00:57AM *  -1 points [-]

For the gears connected to the ground to take energy out of the ground, it has to slow the vehicle down. You are then trying to speed the vehicle up, through the propeller, using only energy derived from the contact with the ground, which is necessarily less than or equal to the energy loss that the vehicle sustained in order to convert its forward momentum into the rotational energy to turn the propeller.

Comment author: wedrifid 23 December 2009 07:11:05AM *  0 points [-]

You are then trying to speed the vehicle up, through the propeller, using only energy derived from the contact with the ground, which is necessarily less than or equal to the energy loss that the vehicle sustained in order to convert that energy into rotational energy.

No, I'm not trying to do that because that wouldn't work. Energy taken from the ground/vehicle difference is not being used to accelerate the vehicle.

Comment author: Nick_Tarleton 23 December 2009 07:04:46AM *  1 point [-]

Gain and loss of energy are frame-dependent; in the road frame, the wind certainly is a source of energy (just as a rocket takes kinetic energy from its reaction mass, when looked at in a frame where it has a greater speed than its exhaust). I'm not sure yet how to think about the vehicle frame.

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