A man is pushing a piano across the floor. What four forces are acting on the piano?

Friction, gravity, normal, and applied force

Explanation:

Friction is the force exerted by a surface as an object moves across it or makes an effort to move across it. Friction depends upon the nature of the two surfaces and upon the degree to which they are pressed together

Gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. By definition, this is the weight of the object. All objects upon earth experience a force of gravity that is directed "downward" towards the center of the earth.

Normal force is the support force exerted upon an object that is in contact with another stable object. For example, if a book is resting upon a surface, then the surface is exerting an upward force upon the book in order to support the weight of the book.

An applied force is a force that is applied to an object by the man to the piano. If the man is pushing the piano across the room, then there is an applied force acting upon the object.