Answer:
When the piano is dropped, the first force acting on it will be the gravitational force, that accelerates the piano at 9.8m/s^2 downwards.
As the piano accelerates, the velocity increases, now appears other force, the air resistance, that opposes to the motion of the piano.
As the velocity of the piano increases, also does the force that the air applies on the piano.
There is a point where the velocity of the piano is such that the air resistance is equal in magnitude, but in the opposite direction, to the force of gravity.
Then the net force on the piano is zero, which means that there is no acceleration, so the piano will keep falling down at constant velocity after this point.
