Me and some friends where down the pub and an interesting idea came up. say a fly is flying and collides with an on comming train then the train and the fly at an instant must stop? how can this be? im trying to think of a way of analysing this. this is my guess. according to newtons third law (object A exerts an equal and opposite force on object B) (i am sure you are aware =] ) then the fly when it collides with the train exerts an equal and opposite force and at that instant stops it? is this true can someone shed some light and help explain this to me =] .