Question

A worker pushes a box with a horizontal force of 40.0 N over a level distance of 4.0 m. If a frictional force of 27 N acts on the box in a direction opposite to that of the worker, what net work is done on the box

Answer 1

Work = (force) x (distance)

The worker does  (40N) x (4m) = 160 joules of work.

Friction eats up    (27N) x (4m) = 108 joules of that energy,
                                                         generating 108  joules of heat.                         

The remaining    (160J - 108J) = 52 joules of energy moves the box.