How do I convert mass units in physics?

[isai]

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.

    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.

 

[mfb]

This seems to be the mass multiplied by the gravitational acceleration g. You don't need that.
The US units have this weird mass/weight confusion where "pound" can be both, the metric system does not.

 

[gmax137]

Many engineers use "kilogram-force" analogous to the US concept of "pounds-force" -- simply because of the convenience when designing bridges which cross rivers on the surface of the Earth (where g=9.8 m/sec^2). In my opinion this isn't a good idea, I think it is mostly older engineers who were schooled in the "customary units" and switched to SI later.

If (when) you get confused, write down F=ma and see that mass has units F/a.

@mfb is right, this can be very confusing and hard to keep straight in your mind. If you are new to physics it would be better to leave this alone for now, and look at it again later after you have done a lot of problems, then it will make more sense.