When a steady net force is applied to an object of rest Mass m0, the object increases in speed. Since the force is acting through a distance, work is done on the object and it's energy increases. As the speed of object approaches c, the speed cannot increase indefinitely since it cannot exceed c. On the other hand, the mass of the object increases with increasing speed. That is, the work done on an object not only increases its speed but also contributes to increasing its mass. Normally, the work done on the object increases its energy. This new twist from the theory of relativity leads to the ideas that mass is a form of energy, a crucial part of Einstein's theory of relativity