Final answer:
The work of friction as a function of time can be expressed as the work input equals the useful work output plus the work done against friction.
Step-by-step explanation:
Friction is a force that opposes the motion of objects in contact. It causes losses of energy, resulting in the work output being less than the work input. In other words, when friction is present, you need to exert more force to overcome it and achieve the desired work output.
The correct expression for the work of friction as a function of time is option B) Work input = useful work output + done against friction.