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A textbook question says that a vehicle weighs $25kg_f$ on Earth, and asks us to consider certain issues related to its behaviour on the moon.

My question is, does the unit kilogram-force $kg_f$ need to be converted to match the gravitational pull of the moon, or is it $25kg_f$ everywhere in the universe?

I read up on $kg_f$ on Wikipedia, but regarding my question, it was inconclusive.

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While there is nothing wrong with the question I'll note that it is generally engineers who worry themselves about $\mathrm{kg}_{f,m}$ (and $\mathrm{lb}_{f,m}$ for that matter), mostly for historical reasons and convenience. Physicists usually understand that you use kilograms for mass and Newtons for force like the SI intended. –  dmckee Nov 11 '12 at 3:22
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up vote 4 down vote accepted

The $kg_f$ is the force exerted by one kilogram at the Earth's surface, so it's numerical value is approximately 9.81 Newton's.

Now 9.81 Newtons is the same everywhere in the universe, so if you consider 1 $kg_f$ to be 9.81 Newtons then it's the same everywhere.

But the force exerted by one kilogram is obviously less on the Moon than on Earth. So a moon dweller might use a unit called $kg_f$ to mean a different force. However this is a different unit. Strictly speaking we should refer to a $kg_{f \space Earth}$ and $kg_{f \space Moon}$ rather than just $kg_f$, to be clear what we're referring to. This possible confusion is why dmckee thinks you should stick to kilograms for mass and Newton's for force. I wholeheartedly agree!

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Thanks John once again for the great explanation! –  Imray Nov 11 '12 at 16:34
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