I am simulating a Cut The Rope style effect on my game, I have a rope which I can cut it when I touch the screen while dragging, but when I make the movement of my finger fast it cannot detect the collision. Anyone please? Thanks

You should save the start point of the finger, and the stop point of the finger swipe. Then you make a raycast between the two points and see if there are any bodies in between. Take a look at the testbed samples (inside RayCastTest) on how to do raycasting.