I am enrolled in a course related with physics, and we have a table of equivalencies of units to get the right units of frequencies. Code (Text): Stiffness Mass Frequency tonf/m ton.s^2/m rad/s kN/m ton rad/s N/m kg rad/s kgf/m kgf.s^2/m rad/s I have always work with the third row, but I want to understand the conversion between mass units only; for example to convert mass from kg to kgf.s2/m. What I have to do it? I have always a problem for this: if I have 50kgf, so to get the mass value for this force, I have to divide by gravity, and I have kgf-s2/m. Furthermore, if I want to convert first to Newtons, then, I have to multiply by 9.80N, and finally divide by gravity to get the mass, and the value of mass is 50kg, different from the other calculation. What have I done wrong? Please help me.