Piano is a very popular instrument widely used in Western music system. It is preferred by many people inspite of the fact that it is an expensive instrument and it is not portable because of its sizes. Piano or Pianofortes is an improved version of Harpsichord which had a single volume and single tone. But Piano plays both loud and soft. Sometimes Piano is classified as percussion as well as a string instrument.
People who consider the Piano as a percussion instrument argue that the strength of the strike on the key determines the volume, that there are many percussion instruments having keyboards and different pitches and that the Piano is as huge and unmovable as other manomoth percussion instruments.
But you should note that only the velocity of the keystrokes determines the volume. The speed of the hammer when it travels between its resting position and the string is responsible for the sound. Keyboard is a means to play specific pitches first like the valves on a trumpet, the frets on a Guitar and a slide on a trombone. Hence it may not be correct to say that the keyboard determines the specific group of any instrument. The following physical aspects of determining the pitch has to be taken into account for classification, viz the valves, the slides, e bows, they keys and the stricks. The classification of Piano has to be based on the sound you hear. It is the string which makes the sound heard from Piano sound board gives perfection and tone to make it a wonderful instrument. In the circumstances, it is right to classify Piano as string instrument, one of the most beautiful string instruments ever invented.