An empty office chair is at rest on a floor. consider the following forces: 1. a downward force of gravity. 2. an upward force exerted by the floor. 3. a net downward force exerted by the air. which of the forces is (are) acting on the office chair?

Explanation:

When a chair is at rest on the floor , the earth will exert a force of gravity  on it which is also known as its weight. This force acts on the chair in downward direction towards the centre of the earth.

The chair also experiences  another force called reaction force which is exerted by the floor on which it rest . This force acts on the chair in upward direction. This force is equal to the weight of the chair  in case chair is at rest or it is moving in uniform motion.

Due to these two equal and opposite  forces, acting on the chair, it remains at rest, as net force becomes zero.

A net downward force exerted by air , which is the reaction force of buoyant force acting on chair in upward direction acts on the floor on which chair rests . This force does not act on the chair.

Hence out of three forces given , only two forces act on the chair.