#### Answer

If you walk on flat ground at constant speed, then the mechanical energy of the backpack does not change. Therefore, the force exerted on the backpack does not do any work.
(1) The force exerted on the backpack by the shoulders and back does negative work.
(2) The force exerted on the backpack by the back and shoulders does positive work.

#### Work Step by Step

If you walk on flat ground at constant speed, then the mechanical energy of the backpack does not change. Therefore, the force exerted on the backpack does not do any work.
(1) If you are walking down steps at a constant speed, the gravitational force pulls down on the backpack as the backpack descends the stairs while the force from the shoulders acts in the opposite direction. Therefore, gravity does positive work on the backpack while the force exerted on the backpack by the shoulders and back does negative work.
(2) If you accelerate, then the backpack accelerates and gains mechanical energy. Therefore, the force exerted on the backpack by the back and shoulders does positive work.