So... you have to measure distance from the start point of the touch to its final touch.
event: when touch starts -> action: save touch's X and Y in global variables
event: when touch ends -> action 1: save distance from those coordinates to the last touch X and Y; use the funcion "distance" like distance(x1,y1,x2,y2)
action 2: use the distance number as a reference to the force applied
Let me know if you get stucked or succeed!