How can it be that my mass is same on Earth and Moon? My weight is different in both, but aren''t those both dependant of gravity? When you stand on a scale, the scale measures your mass according to the pressure you cause to the scale by standing on it, which I think is dependant of the gravity. And that mass should be the same on Moon and on Earth according to my physics book. Weight is gotten from the mass by multiplying it with the falling acceleration g which is different of course in both Moon and Earth, but I still think tha mass should be different too. I know I''m no Galileo, but this is way strange!

Wow, pouya is being informative more & more


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True, but who is to say what mass is the correct one? On Earth, Moon or Mars, it''s never a constant. The "weight" that the scale measures is only based on a deal, which is in the SI system. It''s not an important constant like the speed of light or anything... me thinks. I could be really shitting you people, so tell me if I''m wrong.

quote:Original post by SiCrane

Think of mass as how much matter is needed to balance you on a balancing beam. So in order to measure something 50kg (a person), you stick him on one end of a balancing beam and procede to dump stuff on the other end until you reach equilibrium. So you''ve got 50 kg on both sides. On the moon, while the force on the balancing beam of the person is 1/6 that of on earth, so is the force from the weights on the other side.

This too is wrong. Forces and torques are balanced, not masses. Mass is merely conserved. Weight is the force that balances the balancing beam. You can do a mind experiment to prove this.

Consider that on one side of a beam you have some "mass"---which of course has less weight on the moon and earth. On the other side, instead of a mass you have a spring attached that is stretched to a certain length to directly apply a force. The side with the spring has no mass, but still needs to be balanced. The force of the spring will be the same on the Earth and the moon, since the spring force is a function of how stiff the spring is and how far its stretched. Nothing to do with the mass of the spring. So, if the weight on Earth is enough to balance the constant spring force, then on the moon the weight (which is less than on Earth) will not be large enough to balance the spring force (which is the same on the Earth and moon). Therefore, it is weight and not mass that balances the beam.

If you use the concept of a lever to balance the beam, then you can possibly balance the force of the spring using the lower weight on the moon. It would require that you apply the moon weight at a further distance from the fulcrum of the beam.

Its common in SI units to treat kilograms as a weight, but in fact kilograms are a unit of mass. The true unit of force/weight is the Newton, which is a kilogram-meter per second-squared. In English units, mass is measured in slugs, and weight in pounds, which is a slug-foot per second-squared.

Units are confusing. In English units there is something called a "pound-mass" and "pound-force", and also something called a "poundal". I have never been completely comfortable with any of these. I prefer kilogram and Newton. Slug and pound.



Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

When you stand on a bathroom scale, you are only measuring weight, not mass. Mass is measured on a beam balance, and will show the same mass whether you are on the Earth, Jupiter, or the Moon. Weight is a measure of gravitational attraction, but mass is a measure of how much "stuff" is in an object.

I agree with Myopic and SiCrane.


"Whoever performs only his duty is not doing his duty." --Bahya ibn Pakuda

quote:Original post by CobraA1

When you stand on a bathroom scale, you are only measuring weight, not mass.

True.

quote:Original post by CobraA1 Mass is measured on a beam balance, and will show the same mass whether you are on the Earth, Jupiter, or the Moon.

What are you talking about? What do you mean by beam balance? Without having a way to measure the gravitational constant of the planet/moon in question (something that''s *very* hard to do using *very* expensive equipment---high precision laser instrumentation and such), no balance will truly measure mass. It might have a calculator to divide by 9.8 meters per second squared to report mass to you----i.e., it is calibrated to report mass on Earth----but it will not actually measure mass.

(I think you just didn''t state your thought very well here, .)

quote:Original post by CobraA1 Weight is a measure of gravitational attraction, but mass is a measure of how much "stuff" is in an object.

Yes, of course this is correct.

quote:Original post by CobraA1 I agree with Myopic and SiCrane.

But SiCrane was wrong and contradicted what Myopic said, .

Have a nice day!



Graham Rhodes
Senior Scientist
Applied Research Associates, Inc.

Yes yes, but who did someone who invented kilogram deside how to measure it. How does the scale measure the mass and what makes the certain mass the correct one? It''s damn confusing to think that why is the earth mass correct one and the moon mass not. My brains are not used to this kind of thinking i guess, rather in playing videogames .