i connected the ServoMotor to arduino board (5V and GND and Pin 8)
Just about every servo tutorial shows a servo powered by the Arduino 5V output. That will usually work for one unloaded micro servo. Just about every time someone has a problem with a servo it is because they are powering a loaded servo from the Arduino 5V output. Power the servo with a source that is capable of supplying the current required by the servo under load, around 1A per regular hobby servo. (the Arduino 5V is not capable of enough current).