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In the book (and probably in the movie too, can't remember) there's several mentions of kilograms, with no mention of the difference between kilograms on Mars and on Earth, at least if you try to lift it. Not a huge plot hole, but one has to assume that the great minds at NASA would have some emphasis on the importance of this. Are all masses calculated by volume and density, or do they have special Mars scales around, or what?

The closest mention of this is when he says something like "Yay I'm superman because I can lift like twice as much on Mars as on Earth" (ever so slightly paraphrased).

closed as off-topic by AncientSwordRage Oct 11 '15 at 12:10

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    You can freely convert between the two with surpassing ease – Valorum Oct 11 '15 at 11:05
  • This is much more is a physics question explaining how mass and weight are different than any of the sci-fi elements in the book or film. – AncientSwordRage Oct 11 '15 at 12:11
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    @Richard it's even easier to convert between mass on other planets, given that it's the exact same... – KutuluMike Oct 11 '15 at 12:15
  • Haakon, if you edit the question to make it a bit clearer what you're asking and that it's a sci-fi question rather than a physics question, I'm sure it will get reopened :-) – Rand al'Thor Oct 11 '15 at 12:18
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    Grams are a measure of mass, not weight. On Earth, gravity is used to calibrate scales. On Mars, Martian gravity will be used to calibrate the scale, and a kilogram on Earth will be a kilogram on Mars. An Earth scale on Mars, of course, would give the wrong weight, and would have to be recalibrated. The point is that one kilogram of a given mass will contain the same number of atoms on Mars that it does on Earth. – Howard Miller May 9 '16 at 1:58
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Mass is universal; it's weight that differs according to planet.

The kilogram is a unit of mass, which is a physical property of objects. Something of mass 10 kg will have mass 10 kg anywhere: on Earth, on Mars, or floating weightless in space.

Weight, on the other hand, is a force, also known as gravity. An object's weight, when it's on a certain planet, is the gravitational force pulling it towards the centre of that planet. (There are other gravitational forces pulling it towards everything else in the universe, but these are usually negligible.) Gravitational force is calculated using Newton's law of universal gravitation, but in the simple context of objects on a fixed planet, it can be calculated by multiplying the object's mass by the gravitational acceleration, which is a property of the planet.

An object of mass 10 kg will have weight roughly 98 newtons on Earth and 37 newtons on Mars, the gravitational accelerations on these planets being roughly 9.8 m/s^2 and 3.7 m/s^2 respectively.

  • I'm fairly well aware that 10kgs is 10kgs anywhere, and the difference between weight and mass. I was more curious about how you deal with this when you actually go to another moon or planet. – Haakon Steigre Oct 11 '15 at 11:31
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    @HaakonSteigre In that case, I don't understand what you're asking. Your question mentions "the difference between kilograms on Mars and on Earth", but there isn't one! – Rand al'Thor Oct 11 '15 at 11:33
  • People use kilograms as a unit of weight all the time. Surely your bathroom scale doesn’t read in newtons. – Molag Bal May 9 '16 at 2:16
  • @anaranjada Yes, but thats inaccurate from the physics point of view, which was (I assume) point of the question. Same you could say that you've done a lots of work by bench pressing weights but from the physics point of view you've done no work at all, because at the end of your work weight lies at the same distance from the Earth. Common language =\= physic language. Scales on weight are calibrated by multiplying the force your body presses on the spring * acceleration (9.8) and here you go - you have your mass in kgs – Yasskier May 9 '16 at 4:31

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