Fundamentals of Refrigeration: Weight and Mass

Weight and mass are core refrigeration concepts that are often misunderstood to be different terms that describe the same properties. However, “weight” and “mass” are related, but not identical properties.

Mass is the amount of matter in an object and is measured in pounds-mass, ounces, grams, and kilograms. There is also a lesser-known unit of mass that is called a “slug”. Mass is said to be “conserved,” which means that an object’s mass is the same regardless of where the object is. In other words, an object’s mass is the same whether it is on Earth or the moon.

Weight, however, is a measure of force and is equivalent to the mass of an object multiplied by the gravitational constant. In the United States, the common units used for weight are pounds-force. Weight is not conserved, which means that its value will change based on the gravitational constant used. Since refrigeration applications are employed on Earth, the gravitational constant does not change substantially. This is obvious because a human’s weight does not have a noticeable change at sea level compared to the Rocky Mountains, even though the gravitational constant will be slightly less in the mountains.

Therefore, mass and weight typically have the same numerical value. It is helpful, however, when writing the units to use the subscript ‘f’ for pounds-force and the subscript ‘m’ for pounds mass.

Earlier, it was mentioned that there is a unit of mass named the “slug”. Differentiating between pounds-mass, slug, and pounds-force can confuse even experienced engineers. In the United States, we use pounds as the unit of weight for both force and mass, so it is important to use subscripts to differentiate between the two. The slug is another unit of mass used in the United States.

Newton’s Second Law states that force equals mass multiplied by acceleration. The difference between a slug (mass) and a pound (mass) is the acceleration.

The equation using pounds states that one pound of force equals one pound of mass multiplied by 32.2 ft/s^2. The equation using slugs states that one pound of force equals one slug multiplied by 1 ft/s^2. The difference between the slug and the pound mass is the gravitational constant or the acceleration factor from Newton’s second law.