This question is in response to question B1 in the problems/solutions located at http://aapt.org/physicsteam/2013/upload/E3-1-7-solutions.pdf.
In question B1, there is a wind-powered vehicle that can travel both against the wind and with the wind, and in both cases potentially may go faster than the wind itself.
The force generated by the wind on the propeller is opposite but equal in magnitude to the force the wheel exerts on the ground. In a traditional free-body diagram, one would say the forces balance, therefore the velocity is constant. Moreover, there is loss due to friction and so presumably the vehicle would come to rest.
This, however, is not the case. In the against-wind case, the propellers move through more air than the wheels do ground. The propeller then produces more power than the wheels consume. As a result, the craft speeds up until the frictional loss and the wheels consume as much power as the propeller generates.
It would seem that there must be a restriction on free body diagrams that all forces act through the same distance - or something else of that nature. Is this correct? What restrictions must a free-body diagram have to be guaranteed to be accurate?