Difference between mass and weight

The short answer is: "You're right, it should be in Newtons, kg is a unit of mass, not force, which is what weight is, a force.

Sir Isaac Newton tells us that force is equal to mass times acceleration (F=ma). In the case of weight, the acceleration is gravity at the earths surface (g~9.8 m/s^2). Since g varies little from high altitudes to low altitudes (
Because there was no noticeable difference in weight from place to place, mass and weight was treated as the same thing, since no one knew any better, how would they? Now that we know there is a difference, it is pretty much impossible to get people to change the way they measure, believe me, I know, I'm American and we were supposed to switch to the metric system in the 70s yet I couldn't tell you my weight in kg if my life depended on it.

Hope this helps! We've all been there.

Hey, we are moving to the metric system inch by inch!

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It deals with an elementary argument6 in classical (rational) mechanics. From this point of view, the mass (unit SI = kg) is an INTRINSIC property of the "object", roughly the amount of matter (similar, but not the same, as the amount of substance wich has the SI unit = mol). The weight is a force, which SI unit is correctly the newton. Its normal meaning is the force that a body of mass m exerts on the surface of the Earth: thus (force = mass x accelleration) the weight is the product of m multiplied by the gravity accelleration in that place. So, a body of 80 kg-mass will have, in ANY PLACE OF THE UNIVERSE, a weight of 80 kg-weight: - on average on Earth: 80 x 9.8 = 784 newton - much minus on Mount Everest - more or less 80 x 9.8 x 1/6 = 130 newton on Moon surface - zero when much far fron any celestial body IN ANY CASE 80 kg-weight. P.S.1: when building a balcony or an hammock, take into account the weight, not the mass. P.S.2: a (legal) weighing machine must measure mass, not weight: 1 kg-weight of ham on Mount Everest is minus matter (ham) that 1 kg-weight on sea level ! And I want pay for matter-ham, not for local gravity accelleration !

At least the Metric system is easy to figure out. The english system has the SLUG, what a confusing term.

Yes, we have slugs, poundals, pounds-mass and pounds-force. Confuse your units and you'll be off by a factor of 32.2.

Yes, I was raised in the USA with our educational system focused on "Going Metric", and so we learned both systems. This was very fortuitous because science classes in engineering school required use of metrics almost exclusively, while engineering courses still emphasized the "English" system. However, as a practicing engineer in Manufacturing, I would say my work has required about 90% English and 10% metric, except for the time I worked for the USA branch of a German company, when it became more like 50/50, as I was required to convert a lot of metric data into English for our USA customers. I also worked for a furniture company which used Italian machines, so all of our designs started out in inches and the shop drawings ended up metric!

The greatest advantage of the Metric system is not that a kilogram of mass has a kilogram of weight. We have that too: One pound-mass has a weight of one pound-force. We also have a separate unit of force, like the Newton, called a "Poundal". Acceleration of gravity is about 32 feet per second per second, so one pound-mass will exert a force (or weight) of about 32 poundals, but this is seldom used. Someone also mentioned Slugs, which is also used for mass, i.e. one pound-mass will exert a force (or weight) of 32 slugs. Confused? Many are! That is why we also use a unit-less value for acceleration, the G (one times earth-normal gravity acceleration). One pound-mass accelerated at 2 G's will have a force of 2 pounds-force). Since most commercial engineering calculations (on earth anyway) dealing with loads are not that exact, the difference in weight between one pound-mass at sea level and one pound-mass at higher surface altitudes is not that great.

No, the great advantage of the metric system is that it is a decimal system. If you know the Latin prefixes almost all units convert by simply multiplying or dividing by 10, in other words, just move the decimal point! Of course here in America we say we use the decimal inch as a standard in engineering, designing items using tenths, hundredths, thousandths, and even 10-thousandths of an inch. However, you still see many items designed in fractional inches, 1/2, 3/4, 5/8, 7/16, 1/64, etc. This is primarily because of the huge investment in standard tooling, fittings, etc. and the continual mentoring of younger designers by older ones. The only nod toward decimalization is that you now see fractions converted to decimal-inch, so 1/2 is now .500, 3/8 is now .375 or .38, 7/16 is .4375 or .438 or even .44! No such round off inconsistency in metric. A 5.6 mm design is 5.60 or 5.600 etc, depending on tolerances used or accuracy desired.

Well at least we don't weigh ourselves in "Stones" like the British do. A stone is 14 pounds, ...well, at least out to the 5th decimal place, 13.999998677... go figure!

We have some even more confusing conventions here in the USA. A "2X4" is not 2 inches by 4 inches as the name implies. That is the nominal size of the raw lumber before planing. It is more like 1 1/2" by 3 1/2". A "half inch pipe" isn't a half inch in diameter either. Go figure.