I've lately began working with Newtons laws problems at school again, and i've already ran into a few problems. When making calculations and solving problems, it is often nessecary to understand when forces are equal to zero, and when they are not. Since every force has an equal and opposite counter force, according to Newtons third law, it seems as though the sum of forces are always equal to zero, in both y and x directions. The reason objects are still able to move though, the way i understand it, is because if lets say i push a book across the table, my mass is far greater than the books and therefore my acceleration will be negligible. So it makes sense to me that objects can move, but always using that the sum of forces equal zero when solving problems causes a lot of issues. How do i know when the sum of forces in a direction equal zero, and when they do not?