Why a Grand Piano Keyboard Feels Heavier Than an Upright Keyboard?

The action of a piano is constructed predominantly of components that rotate, such as the keys, whippens and hammers. The response of the keyboard to the fingers is a complex phenomenon that emerges from and is affected by gravity, inertia, friction and the distances between the centers of gravity of the rotating components and their axis of rotation.

When playing, the keys of a grand piano feel heavier than those of an upright, because the inertia related to the rotational movement in the action of a grand piano is greater than that of the mechanism of an upright piano.