The wind that pushes the cart can provide more push than can be harnessed by the cart passively (F=Pressure * area). To increase this force you can either incre

The wind that pushes the cart can provide more push than can be harnessed by the cart passively (F=Pressure * area). To increase this force you can either increase the area of the cart facing the wind, or increase the pressure. The propeller harnesses part of the wind's energy to increase that pressure thus increasing the total force acting on the cart but the work done on the cart (F*displacement) is always limited by the energy that can be provided by the wind.