You lift a heavy box. We’ll consider this process for different choices of system and surroundings.

(a) Choose the box as the system of interest. What objects in the surroundings exert significant forces on this system?

(b) Choose you and the box as the system of interest. What objects in the surroundings exert significant forces on this system?

(c) Choose you, the box, and the Earth as the system of interest. What objects in the surroundings exert significant forces on this system?

The things that are not included in the system is defined as the surroundings and the part that is selected for the consideration is defined the system.

(a)The objects from the surrounding that exerts force on the system are as follows,

The individual and the Earth

The body of the individual exerts the normal reaction force on the box and the Earth exerts the force due to gravity which is also called weight of the object and it is given by,

$W=mg$

Here, m is the mass of the object and g is the acceleration due to gravity.

Thus, the objects from the surrounding that exerts force on the system arethe individual and the Earth.

(b)The objects from the surrounding that exerts force on the system are as follows,

The floor and the Earth

The floor on which the individual is standing exerts the normal reaction force and the Earth exerts the force due to gravity which is also called weight of the object and it is given by,

$W=mg$

Thus, the objectsfrom the surrounding that exerts force on the system arethe floor and the Earth.

(c)The objects from the surrounding that exerts force on the system are as follows,

None of the objects from the surrounding will exerts forceon the system that includes box, the individual, and the Earth.

Thus, nothing will exert force on the system (box, the individual, and the Earth).