My weighting machines notes my weight to be 65. Should I read it 65N or 65kg.

PS: I only need a correct comment.

This question is different, since, I know very clearly what mass and weight are. But very part is that generally students (like me) and people are much more confused that what are they actually measuring in their weight, after they had passed their class 9 and learnt about gravity.

This question can directly solve such query.

  • $\begingroup$ Any measuring instrument should indicate the units it is measuring. Absent those units, you would have to deduce units "based on what makes sense". A person standing on scales reading the number "65" could be pounds (a child or very light adult), kilograms (heavy child, normal adult), but not milligrams, ounces, ... It is EXTREMELY risky and discouraged to guess units when they are not explicitly given. 65 N is a physically unlikely weight for a human on earth (but maybe this was your cat speaking?...) $\endgroup$ – Floris Oct 17 '16 at 12:55
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    $\begingroup$ I believe this question is not a duplicate - the correct answer should address the issue of measurements without units, not the difference between "mass" and "weight". $\endgroup$ – Floris Oct 17 '16 at 12:55
  • $\begingroup$ @Floris Please help me in this chat discussion chat.stackexchange.com/rooms/46614/… $\endgroup$ – Anubhav Goel Oct 17 '16 at 13:24

The term "mass" is an intrinsic property of any body, and doesn't depend on external factors. The term "weight" is a force, i.e. it measures how much a mass is accelerated. Your mass is $m = 65\,\mathrm{kg}$. Your weight on Earth, which accelerates you at $g = 9.8\,\mathrm{m}\,\mathrm{s}^{-2}$, is $$ w \equiv mg = 65\,\mathrm{kg}\times 9.8\,\mathrm{m}\,\mathrm{s}^{-2} = 637\,\mathrm{N}. $$ Similarly, your weight on the Moon is $65\,\mathrm{kg}\times 1.6\,\mathrm{m}\,\mathrm{s}^{-2} = 104\,\mathrm{N}$, and in deep space it's $65\,\mathrm{kg}\times 0\,\mathrm{m}\,\mathrm{s}^{-2} = 0\,\mathrm{N}$, i.e. you're weightless.

Because any acceleration gives you weight, you don't need a massive planet; if you want to visit the ISS, the acceleration of your spacecraft (reaching $29\,\mathrm{m}\,\mathrm{s}^{-2}$, according to NASA) makes you weigh $1950\,\mathrm{N}$ at liftoff, in addition to the $637\,\mathrm{N}$ that Earth makes you weigh. When you get to the ISS, Earth's gravity isn't much weaker than at the ground. There, $g=8.7\,\mathrm{m}\,\mathrm{s}^{-2}$, so if it were hovering above Earth, you'd weigh $565\,\mathrm{N}$. However, since the ISS is in free fall around the Earth, your acceleration with respect to the ISS is $0\,\mathrm{m}\,\mathrm{s}^{-2}$, and you're weightless.

So technically, saying "I weigh 65 kg" is wrong. Instead you should say "My mass is 65 kg". But don't do that. It'll only get you in trouble.

  • $\begingroup$ I think this is an explicit solution to a homework-type question (though it's not 100% clear), which are generally discouraged. $\endgroup$ – Danu Feb 4 '16 at 12:03
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    $\begingroup$ @Danu: From looking at the OP's previous questions, I'm inclined to believe this is a "genuine" question. Anyway, I actually think it's of general interest; I know it took me a long time to get my head around the whole mass/weight/kg/Newton stuff as a student. But now I see that the question has been asked before, which I suppose I should've checked first. $\endgroup$ – pela Feb 4 '16 at 12:45
  • $\begingroup$ Special Thanks to you Pela for such nice and clear answer. $\endgroup$ – Anubhav Goel Feb 5 '16 at 10:36

Your weighing machine measures gravitational force which is your real weight(W=mg) on earth at that point where you are measuring.

And your mass would be according to formula W=mg.


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