THERMODYNAMICS-I

PRESENTATION # 01

MUET SZAB KHAIRPUR MIR’S CAMPUS

AKASH Thahrani

Difference between Pound (mass) & Pound

(force)

POUND

Pound is the unit of mass (weight) and force.

Its abbreviation is lb. lb stands for a Latin word libra which

means Pound. lb first time used in English in late

fourteenth century.

POUND (MASS)

The pound or pound-mass (abbreviations: lb, lbm, lbm) is a unit of mass used in the empirical, United States customary and other system of measurement.

The pound mass is also a fundamental unit within the Imperial system. It is equal to exactly 0.45359237 kilograms by definition.

1 lbm ≡ 0.45359237 kg

POUND (FORCE)

The pound or pound force (symbol: lb, lbf, lbf) is a unit of force in some system of measurement including English engineering units and British gravitational units.

The pound-force is equal to the gravitational force exerted on a mass of one pound on the surface of earth.

Since the 18th century, the unit has been used in low-precision measurements, for which small changes in earth’s gravity (which varies from place to place by up to half a percent) can safely be neglected.

CONVERTING BETWEEN POUNDS-MASS (LBM) AND POUNDS-FORCE (LBF)

If you have an object and you are told its mass in lbm, you can find its weight in lbf, provided you know what the acceleration due to gravity is at that location. Technically, this is not a unit conversion since we are changing what is being measured (from mass to weight), not just the units.

Here is an outline of the steps to find the weight in lbf, given the mass in lbm and the acceleration due to gravity (g).

1. Use Newton’s 2nd Law (F=ma) to find weight in lbm∙ft/s2.

2. Convert weight from lbm∙ft/s2 to lbf using the relationship 1 lbf=32.174 lbm∙ft/s2.THANK YOU