Once we have become comfortable recognizing the forces that act on an object and putting them into a free-body diagram, we can now use Newton’s second law to set up systems of equations that will let us calculate how the forces act on an object relate to the way the object’s motion is changing.  In the same way that we previously learned how to use a vector diagram to set up and solve a vector addition problem, we will learn how to use a free-body diagram to set up and solve a problem using Newton’s second law in the next module.  Because Newton’s second law is fundamentally a vector addition problem, what we now do with our free-body diagrams should feel very similar to working the vector addition problems at the beginning of the course.