Discuss the horse cart problem on level road.

Let the cart be connected to horse through a string. Let  be the tension in the string. To start the motion, the horse presses the ground slantingly and the horse gets equal and opposite reaction 


Let  be the force of friction between cart and ground and let the system accelerate with acceleration 
      Resolve  into two rectangular components 
                 From the free body diagram of horse,
                                     V = mg
and                       H - T = ma                       ...(1)
       From the free body diagram of the cart,
                           T - F = Ma                          ...(2)
Adding (1) and (2), we have,
                          H - F = (m+M)a
or                       
    The system will move if and only if a>0
i.e.                      H>F