In my maths text book, it says that work done can be defined as Force x Distance moved in direction of force, AND change in kinetic energy. I feel both these definitions can be contradictory Example: A box moves at a constant velocity along a rough horizontal plane. It has a driving force of 5N and moves 3m. What is the work done against friction? Well the frictional force is 5N since the box is at a constant velocity. Therefore, work done (against friction)=5N x 3m= 15Nm or 15J (this was the actual example in the book) But, work done is also defined as the change in kinetic energy. This cannot be applied to this example as the box is travelling at a constant velocity so there is no change in kinetic energy. I understand that there are some examples where work done does equal change in kinetic energy, but I don't understand what conditions must apply for this to be true (or not true) How can the definition of work done be one thing under one circumstance and something different in another circumstance? What is the actual, universal, definition of work done that is right in all circumstances?