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Mass and Weight


  • Difference between Mass and Weight
  • Difference between lb, lbf, lbs, lbm

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To calculate cylinder forces we have to operate with mass and weight.

Mass is a fundamental property of the object, a measure of the amount of matter in the object. The “kilogram” (kg) is the SI unit of mass and it the almost universally used standard mass unit. The “pound” (lbm or simply lb) is the unit of mass in the imperial system.  In all scientific work is strongly recommended instead “pound” (lb) use “pound mass” (lbm).

Usually, people write lb when they are talking of one pound or a single pound and write lbs to indicate the fact that they are talking about many pounds. Thus, they use lbs as a plural for lb that stands for a pound. But lb is the correct abbreviation to be used both as singular as well as plural.

If an object has a mass of 1 kg on the Earth, it would have a mass of 1 kg on the Moon, even though it would weigh only one-sixth as much.

1 lbm = 0.45359237 kg

There is also a unit of mass called the slug, defined as the mass which exerts a force of 32.174 049 lbs under the gravitational acceleration at the earth’s surface:


1 slug = 32.174049 lbm

But even in US the use of exclusively SI units for all scientific work is strongly encouraged.

Weight is the force on the object, caused by the gravity and may be calculated as the mass times the acceleration of gravity:

W = F = mg

Acceleration of gravity at the Earth’s surface: g=9.81 m/s2 (approx. 32.174 ft/s2)

The Newton (N) is the SI unit of weight. In the US the pound force unit, abbreviated lbf, is a unit of weight:

1 lbf = 1 slug × 1 ft/s2 = 32.17405 lbm × 1 ft/s2 = 32.17405 lbm × ft / s2

1 lbf = 0.453 592 37 kg × 9.80665 m/s2 = 4.44822162 N

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