A particle moves in the x-y plane under the constraint that its velocity is always directed towards a point on the x-axis whose absicissa is some given function of time f(t). Show that for f(t) differentiable, but otherwise arbitrary, the constraint is non-holonomic. All I could infer from the above question is: x = Cf(t) C is a constant. If the velocity is directed towards a point on the x-axis, is the same point? Could someone guide me in the right direction?