I am enrolled in a course related to physics, and converting between mass units is so tricky for me. I always treat in my work only in SI units and maybe MKS units, at level of weights, I find it easy.

But what about the mass conversion between these units:

  • kg
  • kgf.s^2/m

The first I see is that kg comes from N, and kgf.s^2/m comes from kgf, doesn't it? I don't understand, how to convert, for example 50 kg to ()kgf.s^2/m; I couldn't affect it by gravity because they are mass units, not weight-mass conversion anymore.

Please help me.

This is my notes from my class, to get the right units of frequency:

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 chose the third row to calculate; but maybe in the exam it could come in other units; and I want to understand it.

I don't know if this is so basic that I have to know, but don't get it.

  • $\begingroup$ I don't know who your instructor is, but he needs to be aware that there are no such units as kgf. If you are in college, and past the drop date, good luck. $\endgroup$ Commented Oct 5, 2018 at 16:37
  • $\begingroup$ As you comment and suggest, I always work with SI units; but I am afraid of the test about this, because I don’t understand how to convert. And, as I posted another question, I am unable to continue after the relation of mass units. $\endgroup$
    – Isai
    Commented Oct 5, 2018 at 16:41

1 Answer 1


You said that kg comes from N, that's wrong, the value of the kilogram is different to the value of de N (Newton). If you want to calculate the Newtons of a force being applied to a mass you must use F=m*a. Kilograms is the mass and a is the acceleration. As an illustration, LET'S USE earth's gravity to calculate the force that you receive according to your mass. Let's say you have a mass of 50kg, and the gravity (acceleration) on earth is 9.8m/s^2. The force in Newton will be F = 50 * 9,8. And Newton in its formal definition is kg·m·s-2.

And 1 kgf (kilogram-force) is just equivalent to 9.8 N.

  • $\begingroup$ Thank you so much; but, how to convert 50 kg to ()kgf.s^2/m? Furthermore, when I mentioned that kg comes from N, I meant that is a conversion from weight in N to kg. The first question please, or kgs-s^2/m is not mass unit? $\endgroup$
    – Isai
    Commented Oct 5, 2018 at 9:09
  • $\begingroup$ kgf-s^2/m cannot be a mass unit, the s stands for seconds, therefore it is not a mass. But i still don't understand what do you mean by kgf-s^2/m. The kgf is equivalent to Newton which in its formal definition is kg·m·s-2. The unit kgs-s^2/m does not exist, i think you probably got it wrong while taking notes. $\endgroup$
    – user208706
    Commented Oct 5, 2018 at 9:15
  • $\begingroup$ I thought like you, but I didn't ask in class about it, because I thought that I figured it later. But, when you want to convert weight in kgf to mass, you have to divide it by gravity acceleration in our case, and the units will be kgf-s^2/m, isn't? $\endgroup$
    – Isai
    Commented Oct 5, 2018 at 9:20
  • $\begingroup$ Okay, now i get your question! Sorry! If you have the weight in kgf and you want the mass in kg you just use the F = m*a, because remember that 1kgf = 9.8N, you just need to do m = F/a. Where the force would be kgf and a 9.8 m/s^2. $\endgroup$
    – user208706
    Commented Oct 5, 2018 at 9:27
  • $\begingroup$ I added a table from my class notes, the goal is to get the right frequency units. Are the units right? $\endgroup$
    – Isai
    Commented Oct 5, 2018 at 9:30

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