So far, our picture of natural forces as being the cause of the acceleration of mass seems fairly successful. In time it will become second nature to you; you will intuitively seek a force behind all forms of change. However, our description thus far is fairly simplistic - we have massless strings, frictionless tables, drag-free air. That is, we are neglecting certain well-known and important forces that appear in real-world problems in order to concentrate on ``ideal'' problems that illustrate the methods simply.
It is time to restore some of the complexity to the problems we solve. The first thing we will add is friction.