Trabalho realizado em um sistema por uma força externa

Consider a person moving a box in a warehouse. If there is no frictional force, the work done by the applied force on the box would be the change in the kinetic energy of the box. In real life, a frictional force acts on the box. So, the difference between the applied force and the frictional force magnitudes decides the acceleration of the box. As the forces are constant, the box has a constant acceleration. The final velocity can then be written in terms of the box's initial velocity, acceleration, and displacement. The acceleration is made the subject, and its expression is substituted into the force equation and further simplified. As there is no change in potential energy, the difference in kinetic energy can be written as the change in the mechanical energy of the box. The second term represents the increase in thermal energy due to friction between the box and the floor. In summary, the work done by external forces equals the energy transferred to or from the system.