I have tried to find the least confusing (or rather, the most 'deconfusing') explanation for why it is possible for a cart with a prop to travel directly downwind faster than the wind. So far this is what I've come up with: Initial discussion At first glance, a cart travelling down faster than the wind would experience a headwind. The drag of the headwind would slow down the cart. However, if that cart also has spinning blades, the back side of the blades could run into the air, such that the pressure on the blades is from behind, not in front. So this would have the effect of applying a forward pressure on the vehicle which would force the cart, prop, bearings, etc. to move forward. Because the bearings hold the axle, the increased translational velocity of bearings relative to the ground would cause the wheels and axles to spin. So at the expense of some of the gained translational kinetic energy of the cart and bearings, the wheels and axles will gain rotational kinetic energy in addition to their own translational kinetic energy. Because the prop is connected to the axle via a belt, the prop must then recover rotational kinetic energy at the expense of the rotational and translational kinetic energies of the rest of the cart (as well at the expense of some of the prop's own translational kinetic energy). This gain of rotational kinetic energy by the prop must overcome the loss of rotational kinetic energy due to the drag by the air that it slices through. Revealing the hidden assumption When we say the cart is travelling directly downwind faster than the wind, we are saying that both the cart and the wind have a speed, but one is greater - relative to something. Strictly speaking, the norm of the velocity difference between the cart and this something is greater than the norm of the velocity difference between the air and this something. In the initial discussion, we choose this something to be our ground. Then we say the cart is travelling directly down the wind faster than the wind as a result of work done on it (relative to the ground) and that kinetic energy of the wind (relative to the ground) is extracted upon by the cart to provide this work. Forces or work, which is more important? Ignoring relativity, the forces do not change with the frame of reference, but the displacements do. Therefore the work done on or done by the air, ground, cart, the parts, etc. is dependent on the inertial frame of the independent observer. So if we choose an observer independent of the ground, then work may be done by the air on the cart, or vice versa, and work may be done by the ground on the cart, or vice versa. However, if by displacements we meant displacements relative to the initial rest frame of each material entity and not displacements relative to an arbitrary inertial observer, then given a reasonable assumption that the ground has a very large inertia relative to the cart and the air, the ground may be seen as receiving no work upon it, according to its unchanging rest frame. The cart may be receiving a force according to its frame, but the rate at which work is done on the cart it is ill defined in this frame unless an initial rest frame for the cart can be chosen. If this could be done for both the cart and the air, then one may say the air does work on the cart (or vice versa), depending on the initial "rest" frames for the air and the cart. Lacking specification of these initial rest frames, then the transfer of energy between the air and the cart and between the ground and the cart is frame-dependent. So it is not so much that the wind does work on the cart, or the ground does work on the cart, or whatever combination one so desires, since that is the arbitrary result of choosing a frame of reference. In the end, we are dealing with differences in velocity and their norms (i.e. speeds relative to ground), and therefore differences in momentum and their derivatives with time (i.e. forces). Therefore, the deciding factor that makes DDWFTTW possible are the forces on the bodies and not work done between them. If one insists to use an work-based or power-based explanation, they should clearly specify the frame of reference in which the kinetic energies are defined, and if they are going to analyze the problem from different frames of reference, they should make it clear that kinetic energies and powers are not the same between different frames of reference. If they jump between different frames of reference when thinking about the DDWFTTW in terms of kinetic energies and powers, they will likely confuse themselves if they are not careful. Sincerely, Kevin M.