PE decreases when altitude decreases, so that work is done on the object by gravity.  The work done on the object against the net force will tend to increase its KE.

• As its gravitational PE decreases by 82 J, gravity does 82 J of work ON the object, which is equivalent to negative work being done BY the object .
• At the same time 57 J of work is done ON the object by all other forces, equivalent to the - 57 J of work done BY the object against those forces.
• Thus the object does - 57 J - 82 J = -139 J of work against all of the forces acting on it--i.e., against the net force on it. This is the net work done BY the object.
• As the object does negative work its KE change is equal to the negative of the work done BY the object, so KE change is --139 J = 139 J.

As an example consider an object falling under the influence of gravity.

• Its gravitational PE decreases, tending to increase its KE.
• If the object is subject to an additional downward force whose magnitude is greater than that of the air resistance, the net nongravitational force will be in the direction of motion. 
• Work in addition to the work done by gravity is therefore done on the object by the nongravitational force, tending to further increase its KE.