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I have a problem converting force to mass units, in this example for 9.81kgf:

  • To get the mass, dividing by gravity, I get 1kg of mass.

  • If I want to convert to Newtons first, I have to multiply by 9.81N, then divide by 9.81m/s^2; finally I get 9.81kg of mass.

The results are different, what have I done wrong? I don't get it.

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  • $\begingroup$ Possible duplicate of Conversion mass between mass units $\endgroup$ – Aaron Stevens Oct 5 '18 at 13:26
  • $\begingroup$ I don't have answers. Sorry, but this is more specific. $\endgroup$ – Isai Oct 5 '18 at 13:27
  • $\begingroup$ 9.81 kgf is already 9.81 N. $\endgroup$ – Rodney Dunning Oct 5 '18 at 13:55
  • $\begingroup$ But, for definition 1 kgf = 9.81 N. $\endgroup$ – Isai Oct 5 '18 at 14:05
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    $\begingroup$ @Isai, there is no SI unit for kgf, and there is no conversion factor for it. Mass is kg. Force is Newtons. I suggest you carefully read chapter 1 of practically any high school physics book. $\endgroup$ – David White Oct 5 '18 at 16:32
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One $kgf$ is the force produced by one kilogram at the Earth's surface, that is when the gravitational acceleration is $g$, so $9.81 kgf$ is the force produced by $9.81$ kilograms.

You say:

To get the mass, dividing by gravity, I get 1kg of mass

and you are correct that you need to divide by the gravitational acceleration, but you have to keep your units consistent. If your unit of force is kgf then the unit of acceleration is $kgf/kg$ and that means in these units $g=1$. So the mass corresponding to $9.81~kgf$ is $9.81/1 = 9.81$ kg.

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