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I feel like I am missing something fundamental with regards to mass vs. weight.

The weight of an object on the Moon is approximately 16.5% what you would experience on Earth.

In other words, if an object weighs 100kg on Earth, it would weigh only 16.5kg on the Moon.

As I'm led to believe by this statement, weight is variable, and depends on the gravitational force exerted upon it, but it's the same object, with the same constant mass.

I am even more confused by the statement made in the following site: https://www.mathsisfun.com/measure/weight-mass.html

An object has mass (say 100kg). This makes it heavy enough to show a weight of 100kg.

What I am confused about here, is the statement "An object has mass (say 100kg)", surely that is based on its weight on Earth?

  • Is it correct to say that an object has a constant mass of 100kg, and weighs 100kg on Earth, but only 16.5kg on the Moon, and if so, why (what I'm getting at here is, do we use Earth's gravitational force as the reference point whereby an object's mass is equal to its weight)?
  • More generally, if the statement above is incorrect, is there a mechanism for calculating the constant mass of an object, and if so, what is it, and what is it measured in?
Matthew Layton
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    Weight is a force and should really be measured in Newtons. – ZeroTheHero Mar 17 '23 at 14:54
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    Who are the total remtards downvoting this question? Guy comes along and asks a perfectly valid question and gets downvoted. And people wonder why Stack Exchange is considered such a toxic community these days? Grow up people. Either help or jog on. – 0b101010 Mar 17 '23 at 15:49
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    It happens to everyone. Hard to say why. It isn't always deserved. Shake it off. – mmesser314 Mar 17 '23 at 15:57
  • @0b101010 likely because questions like this have been asked numerous times and the asker hasn't even shown that they've searched the site for possible duplicates or related questions. – Triatticus Mar 17 '23 at 20:05
  • @Triatticus Actually, I did look at similar questions. There was one with exactly the same title, but the answers to those questions were insufficient, hence the accepted answer below. 0b101010 makes valid points, both in their answer, and their comment, above. – Matthew Layton Mar 18 '23 at 08:01
  • Such information should be provided in the question and possible reasons yours is different from those similarly titled, or why similar questions don't already answer your question. – Triatticus Mar 18 '23 at 09:50
  • @Triatticus i think it would be entirely unreasonable to expect that. Either contributors to this site are willing to help people (despite potential reposts or inexactly worded questions) or there're not. And if they aren't then what's the point of the site. Seems to be a lot of sanctimonious people here fluffing their own egos by putting other people down. That's pretty nauseating behaviour on a site that should welcome people curious about physics and wanting to learn. – 0b101010 Mar 18 '23 at 21:11

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The confusion here stems from the fact that popular culture refers to weight using kilograms (e.g. your bathroom scales). This is wrong. Totally wrong. The internet is full of websites that routinely get this wrong. Even ones that pretend to teach physics.

Mass is a measure of an amount of stuff. An apple on the Earth has the same mass if you move it to the Moon. The SI unit for mass is kilograms.

Gravitational fields are described by the amount of force experienced per unit of mass at a particular point in the field. This is measured in N/kg.

Weight is the force a certain mass experiences at a particular point in a gravitational field. Since it is a force it's measured in N.

As you correctly summise, the confusion arises because the Earth's gravitional field is pretty much constant across its surface and therefore a direct mapping of weight to mass exists for measurements conducted at the surface. Your bathroom scales are calibrated using this assumption. Take those scales to the Moon and they will incorrectly indicate your mass has reduced. Unless you've lost a limb or two on the journey there, this will not be true.

0b101010
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