If you understood the first two chapters, you now know how to describe motion (using velocity and accelerations) and you know what causes changes in motion (net forces do it). Remember that net force refers to what you get when you consider the total effect of all the forces acting on something. If two equal forces are acting in opposite directions, the net force is zero. And of course you remember that an acceleration is any change in speed and/or direction. So a net force acting on an object causes the object to accelerate. All this knowledge makes you a junior science nerd, so why not try for apprentice nerd by learning exactly how forces and acceleration are related.