Springs without mass, that satisfies the Hooke’s law. xi and xf are, respectively, the initial and end length that the spring is stretched or compressed. If the spring is stretched the length is positive, if is compressed is negative. Allow one or two forces. If have two forces these must have different sense (stretching or compressing the spring), and the same values or unknown data. Don’t allows frictions or contacts. The forces must be in the direction which will move the spring (see examples).