Mass versus weight

Do not confuse mass with weight in the metric system.  Weight is a measure of force.  In the imperial system of measurement, units such as pounds, tons, scruples, drams, ounces and grains are dependent on gravitational pull and so are designated as measures of weight (and not mass).  The unit of measure for mass in the imperial system is the slug.  Gravity has no bearing on a slug.  On the surface of the Earth one slug is equivalent to 32.174 lbs.  This means that a fellow weighing 200 lbs on Earth has a mass of exactly 6.216 slugs.  Put him on the Moon and he weighs less in pounds but still has a mass of 6.216 slugs.

However, you can (if you confine all of your figuring and calculations to activities on the surface of the Earth [at sea level, not down in the Marianas trench nor up on Everest], giving you an unchanging environment for the pull of gravity) equate mass and weight.  In 1959 a joint organization representing the national laboratories of the U.S., New Zealand, Canada, South Africa, England, Wales and Australia defined imperial units of measures of pounds, tons, ounces and the rest to be allowed to act as units of mass (in addition to being units of measures of weight).  So academically, mass and weight are different measures, but by agreement, they can be the same under constant gravitational conditions without the thought of travelling to where the acceleration of gravity differs from 32.174 feet/second² and measuring things there.  So, since 1959, if you are aware of what you're saying, you can equate pounds and kilograms as mass.  But most people mistakenly do so.

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