Tuesday, 2 October 2018

newtonian gravity - What is my real weight?



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.



Answer



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.


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