## Mass and Weight

Summary:

- 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**” (**lb _{m}** 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**” (

**lb**).

_{m}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 **lb _{m}**

**=**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 **lb _{m}**

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/s^{2} (approx. 32.174 ft/s^{2})

The **Newton** (**N**) is the SI unit of weight. In the US the **pound force** unit, abbreviated **lb**_{f}, is a unit of weight:

1 **lb**_{f }= 1 slug × 1 ft/s^{2} = 32.17405 **lb _{m}** × 1 ft/s

^{2}= 32.17405

**lb**

_{m}× ft / s^{2}1 **lb**_{f} = 0.453 592 37 kg × 9.80665 m/s^{2} = 4.44822162 **N**