There are lot of examples given in a physics text books on vibration. There is a spring and a mass is shown at the end of the spring. In reality i am not seeing a spring and a mass separately. For example consider a rod it has stiffness of a spring and has a mass integrated in one object. So how this spring separate and mass separate hypothesis works out. In some other problems a spring contains a mass at one end and the other end of the mass is connected to the other end. Where this kind of approximation will simulate reality?