Pound

:This article is about the unit of weight. For other uses, see Pound (disambiguation).

Force, weight, and mass

Historically, the pound predates the understanding of the distinction between force and mass. Once that distinction became clear, it was natural to ask whether the pound should be construed as a unit of mass, or a unit of force. But because the foot-pound-second systems are no longer used in science (and are gradually approaching extinction even in U.S. engineering work), many scientists today would be as bemused by this question as by the question of whether the shekel is a unit of mass or of force.

Related Topics:
Force - Mass - Shekel

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In many contexts, there is a long history of considering the pound to be a unit of mass:

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• Pounds were primarily a measure of how much stuff people had, for the purposes of trade. We know what we use for those purposes today?the only pounds legal for trade anywhere in the world are those defined as units of mass exactly equal to 0.45359237 kg.{{fn|1}}
• Mass-measuring balances were the only weighing instruments anybody ever used before the 19th century.{{fn|2}}
• Over time, the various keepers of the standards redefined pounds in terms of the metric system (which has happened in case of the avoirdupois and troy pounds as well as the metric pounds), they were defined in terms of the kilogram, not the dyne or the newton.
• When units such as the British thermal unit are defined based on the pound, those pounds are units of mass just like the grams or kilograms used as the basis of the definition of calories.
• On labels of products sold in the United States, the pounds and ounces are units of mass, like the grams and kilograms which appear right alongside them.

On the other hand, pounds are always to be construed as a unit of force in contexts such as these:

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• Thrust of rocket or jet engines in pounds-force.
• Torque in foot-pounds or pound-feet.
• Pressure in pounds per square foot or pounds per square inch.
• Energy in foot-pounds.