# Def'n: mass vs. weight / kg vs. lbs

D H
Staff Emeritus
http://www.google.com/search?num=10...I7GGLR&q=2.7+kilograms+in+pounds&btnG=Search" link, you guys keep referring to is only true on the earth! I'M REFERING TO THE MOON!! No crap that 2.7 kilograms on the earth weighs 6 pounds BUT ON THE MOON 2.7 kilograms weighs 1 pound. And if you took measurements from another plant the value would change again. That's all I'm saying.
What you are saying is wrong because pounds are units of mass, just as are kilograms. You are confusing pounds with pounds-force. They are different things. People use the term "weigh" colloquially when they mean "mass". If you ask someone in England or Canada how much some object weighs, they will give an answer in kilograms, not newtons.

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D H
Staff Emeritus
Gee, I't's good to know that at least someone remembers the slug. Last time I checked (and admittedly that was 20 years ago), the slug is THE unit of mass in the US customary unit of measure, as used in Physics. . In Newton's second law, a 1 pound force is defined as that force necessary to accelerate an object with a mass of 1 slug at a rate of 1ft/sec/sec. And since W=mg, a 1 slug mass weighs 32 pounds, more or less, on Planet Earth. Any other way of defining mass in the US units will throw you for a loop when solving Physics problems in statics or dynamics using Newton's laws. And since metric, first introduced to the US was it 40 years ago, has all but died in the US (even NASA issues a big exception to the exclusive use of SI, and the FEDS have given up trying to make the transition), the slug is here to stay, like it or not, for another 40 years or more.
You can have F=ma and use pounds for mass. You just have to express force in poundals rather than pounds-force.

PhanthomJay