the lenght of ladder is contant. when it is in the middle of sliding, it make a right angle triangle with the floor and wall, hence (the lenght of the ladder£^2 equal to (the lenght of floor to one end of the ladder)^2 + (the lenght of wall to the other end of ladder)^2. to simplify it will look like a^2 + b^2= constant^2.. which is circular motion.