Work in free expansion I have read that no work is done during free expansion and temperature of an ideal gas undergoing free expansion doesn't change. 

Let us assume an ideal system of piston and gas which has vacuum around it. The gas will expand but we say there is no work done by the gas. Why so ? Gas applies a pressure(&amp; hence a force) on the piston and the piston gets displaced. So acc to F.s = work , there is work done by the gas !
Little bit tricky question. As you say, the work is force acting along the path. But there is no force acting, as there is weightless piston with vacuum on the opposite side.

To visualize the process better, imagine an inflatable balloon in outer space, when you puncture it. All gas molecules continue to fly in the direction and speed they had in the moment of accident (therefore the temperature is the same) and they exert no work (no collision thereafter).