introduction to mechanical engineering lesson 6 intro eng1

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Introduction to Mechanical Engineering

Lesson 6 Intro Eng

Lesson 6 Intro Eng 2

Mechanical Engineering Topics

– Work, force and distance– Materials science– Statics & Dynamics– Fluid Mechanics – Thermodynamics– Practical heat– CAD design


Force • Forces acting on objects are vectors that are characterized by not only a magnitude

(e.g. pounds force or Newtons) but also a direction. A force vector F (vectors are usually noted by a boldface letter) can be broken down into its components in the x, y and z directions in whatever coordinate system you’ve drawn:

• F = Fxi + Fyj + Fzk• Where Fx, Fy and Fz are the magnitudes of the forces in the x, y and z directions and

i, j and k are the unit vectors in the x, y and z directions (i.e. vectors whose directions are aligned with the x, y and z coordinates and whose magnitudes are exactly 1 (no units)).

• Forces can also be expressed in terms of the magnitude = (Fx2 + Fy2 + Fz2)1/2 and direction relative to the positive x-axis (= tan-1(Fy/Fx) in a 2-dimensional system). Note that the tan-1(Fy/Fx) function gives you an angle between +90 and -90 ̊ ̊whereas sometimes the resulting force is between +90 and +180 or between -90 ̊ ̊ ̊and -180 ; in these cases you’ll have to examine the resulting force and add or ̊subtract 180 from the force to get the right direction. ̊


Fundamentals of Materials

• Materials science is about synthesis, fabrication and processing materials. The science of molecular bonding and structure are a major consideration and materials science relies upon thermodynamics for the behavior of difference materials under different conditions. It is usually suggested that thermodynamics is taken first.

• Three topics fundamental to materials science and engineering are structure, bonding, and thermodynamics. These topics are not traditionally taught in the same course or in tandem, though a structure course and thermodynamics course are usually part of the first core courses taken in a Materials Science and Engineering curriculum. However, modern engineering practice requires that these conceptual ties between these subjects be brought together. Bonding dictates structure, and structure in turn provides constraints on the thermodynamic properties of materials. These topics are intimately related and a full understanding of materials' synthesis, fabrication, and processing (the essence if materials science) relies on bringing out these interconnections. In addition, it is enlightening to learn about the same materials from different viewpoints, to better appreciate the diverse perspectives we take when looking at materials science: What is the crystal structure of diamond? How does it affect its thermodynamics properties? How is it related to the nature of the bonding between carbon atoms? One then begins to see how these fundamental properties of materials are connected. Materials is a favored course in mechanical engineering.


Statics and Dynamics

Statics is a Review of Vector Algebra; Principles of Mechanics; Free-Body Diagrams; Static Equilibrium of Particles and Rigid Bodies; Distributed Force Systems. Structures, Beams, Trusses, Frames, and Cables; Friction; Center of Mass, Centroid, and Moment of Inertia. Dynamics is a Review of Particle Dynamics; Free-Body Diagrams; Force, Energy and Momentum Methods; Planar Kinematics and Kinetics of Rigid Bodies; Energy Methods; Vibration Problems.

See Handout

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Fluid Mechanics

• Fluid mechanics is just ΣF = d(mu/dt) (Newton’s 2nd Law, the sum of the forces is equal to the rate of change of momentum) applied to a fluid. For our purposes, what distinguishes a fluid from a solid is that a solid deforms only a finite amount due to an applied shear stress (unless it breaks), whereas the fluid continues to deform as long as the shear stress is applied. This makes fluid mechanics a lot more complex than solid mechanics.

Lesson 6 Intro Eng


First Law of Thermodynamics • The First Law of Thermodynamics says the energy contained in an isolated system (one that does

not exchange energy with its surroundings) cannot change. Of course, energy can be converted from one form to another, which is the whole point of energy engineering. However, the sum result is constant.

• Quantitatively, the 1st Law of Thermodynamics for a control mass, i.e. a fixed mass of material (but generally changing volume, for example the gas in a piston/cylinder) can be stated as follows:

• where• ΔE = ΔQ – ΔW• E = energy contained by the mass - a property of the mass (in BTU) Q = heat transfer to the mass

(in BTU) W = work transfer to or from the mass. Δ = Change from state 1 to state 2; thus the 1st law could also be written

• E2 –E1 =Q1→2 –W1→2• i.e., the change in energy contained by the mass is equal to the heat transferred to the mass

minus the work transferred out of the mass. Work transfer is generally defined as positive if out of the control mass, in which case - sign applies, i.e. ΔE = ΔQ - ΔW; if work is defined as positive into system then ΔE = ΔQ + ΔW. A process in which no heat transfer occurs is called an adiabatic process.


Second Law of Thermodynamics • For our purposes, the Second Law invokes a property of substances called

entropy, which is the measure of the “disorganization” or “randomness” of a substance. The hotter or less dense a substance is, the less information we have about where the individual molecules are, and thus the higher its entropy will be. The Second Law can be stated simply as

• The entropy of an isolated system always increases or remains the same• meaning that the entropy never decreases. The methane – air mixture at

300K has a lower entropy than the carbon dioxide, water and nitrogen mixture at 2000K, so only the usual combustion process is physically possible, never the reverse. (Of course I could take that carbon dioxide, water and nitrogen at 2000K, cool it off to 300K, break the molecules apart, rearrange them to form methane and air, but to do this I would need to increase the entropy of the surroundings by more than the entropy change of combustion, so there would be a net increase in the entropy of the universe.)


Basics of Heat

– Fundamentals of Heat • Practice of heat.

– Boyle’s Law:» Purpose: To gain and understanding of pressure, volume

and temperature in a closed vessel. » Purpose: to calculate basic proportion and variation » Purpose: to understand the thermo-properties of gas

– Expansion and contraction of materials:» Coefficient of expansion and contraction – see tables » Specific heat of substances – see tables

– Temperature scale conversions – see formula


Computer Aided Design

– CAD Design• Goal: To be able to properly utilize Computer Aided

Design instruments• Cover: CAD design using software such as Autodesk®

InventorTM Professional, MasterCAM® or similar software available at UHCC.• Lab Assignments:

– “Simple Shapes”-cylinders, squares, circles, etc.» Purpose: Student will learn the basics of CAD design and

utilizing UHCC CAD software.

introduction to mechanical engineering lesson 6 intro eng1

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