Mechanics, science concerned with the motion of bodies under the action of forces, including the special case in which a body remains at rest. Of first concern in the problem of motion are the forces that bodies exert on one another. This leads to the study of such topics as gravitation, electricity, and magnetism, according to the nature of the forces involved. Given the forces, one can seek the manner in which bodies move under the action of forces; this is the subject matter of mechanics proper.

When a spring vibrates every parts of it don't vibrates with same velocity and therefore have different kinetic energies. One can't therefore added it to the loaded mass directly while determining the frequency of the oscillation. Therefore the concept of effective mass has been introduced. For an ideal spring the effective mass is 1/3rd of the total mass of the string. If accuracy is not very important in certain situation one can consider mass of the spring to be zero.

Sir, what is the actual meaning of the effective mass of a spring?