CHAPTER 9 MASS AND MASS-RELATED PARAMETERS

Introduction Mass is another important fundamental dimension that plays an important role in engineering analysis and design. Knowledge of mass is important in determining the momentum of moving objects. In engineering, to represent how light or heavy materials are, we use properties such as density and specific gravity. In this chapter, we will introduce the concept of mass in terms of a quantitative measure of the amount of atoms possessed by a substance. We will then define and discuss other mass-dependent engineering quantities, such as density, specific gravity, mass moment of inertia, momentum, and mass flow rate.

Table 6.7 is repeated here to remind you of the role of fundamental dimensions and how they are combined to define mass-dependent variables such as moment of inertia, momentum, and mass flow rate

Mass as a Fundamental Dimension Humans noticed that some things were heavier than others and thus recognized the need for a physical quantity to describe this. All objects and living things are made of matter, and matter itself is made of atoms, or chemical elements. Mass provides a quantitative measure of how many molecules or atoms are in a given object. The matter may change its phase, but its mass remains constant. mass moment of inertia: A measure of how hard it is to rotate something with respect to center of rotation. Mass also plays an important role in storing thermal energy. Some materials are better than others at storing thermal energy

Measurement of Mass The kilogram is the unit of mass in SI. In the British Gravitational system of units, the unit of mass is slug, and in U.S. Customary system of units, the unit of mass is lbm . (1 kg = 0.0685 slugs = 2.205 lbm and 1 slug = 32.2 lbm). In practice, the mass of an object is measured indirectly by using how much something weighs. The weight of an object on earth is the force that is exerted on the mass due to the gravitational pull of the earth (F=mg).

Mass Flow Rate The mass flow rate is another closely related parameter that plays an important role in many engineering applications. Mass flow rate tells engineers how much material is being used or moved over a period of time. Engineers use flow-meters to measure volume or mass flow rates of water, oil, gas, chemical fluids, and food product The mass flow rate is simply defined by the amount of mass that flows through something per unit of time.

Conservation of Mass In the analysis of engineering work we need to keep track of physical quantities such as mass, energy, momentum, and so on. Let us now look at how engineers may go about keeping track of mass and the associated bookkeeping procedure (see Figure below). Simply stated, the conservation of mass says that we cannot create or destroy mass. (The rate at which water enters the container ) – (the rate at which water leaves the container ) = (the rate of accumulation or depletion of the mass of water within the container)

Example

قوة دافعة

Kinetic energy