The solution for fh has a singularity (that is, becomes infinitely large) at a certain angle θcritical. For any angle θ>θcritical , the expression for fh will be negative. However, a negative applied force fh would reverse the direction of friction acting on the lawnmower, and thus this is not a physically acceptable solution. In fact, the increased normal force at these large angles makes the force of friction too large to move the lawnmower at all.