Mechanical Engineering as a Profession 1、 Introduction Scientists study the world as it is, engineers create the world that never has been What is engineering Engineering is the art of applying scientific and mathematical principles, experience, judgment and common sense to create things that benefit people In other words, engineering is the process of producing a technical product or system to meet a specific need What engineers do Engineers: Turning Ideas into Reality Engineers do practical design and production work using the principles of science and mathematics to come up with economically viable solutions to technical In shorter words, they design and build needed tools, products, and systems What is mechanical engineering Mechanical engineering is the most diverse and exciting of all the engineering disciplines It is specifically concerned with the design, development, installation, operation and maintenance of just about anything that has moveable parts If an object is man-made, mechanical engineering skills will have been involved at some stage during its development and manufacture Role of Mechanical Engineers We Make Machines.. and More Mechanical engineers are professionals devoted to employing the principles of motion, forces, and energy. Mechanical engineers research, develop, design, manufacture test tools, engines, machines, and other mechanical devices. For society this means that mechanical engineers are using their wide range of skills to think of ways to 2、Top10 Benefits The top 10 rewards and opportunities that an engineering career offers #1 Job satisfaction It is important to find a career that provides you with enjoyment and satisfaction After all, you might spend 40 or so years working eight hours or more a day, five days a week, 50 weeks a year. Do you want to dislike every minute of that time, or would you rather do something that you #2 Varied opportunities An engineering degree offers a wide range of career possibilities If you are imaginative and creative, design engineering may be for you. If you like laboratories and conducting experiments, you might consider test engineering If you like to organize and expedite projects, look into being a development engineer If you are persuasive and like working with people, consider a career in sales or field service engineering The analytical skills and technological expertise you develop as an engineering student can also be put to use in many other fields
Mechanical Engineering as a Profession 1、Introduction "Scientists study the world as it is, engineers create the world that never has been.” What is engineering ? Engineering is the art of applying scientific and mathematical principles, experience, judgment, and common sense to create things that benefit people. In other words, engineering is the process of producing a technical product or system to meet a specific need. . What engineers do ? Engineers: Turning Ideas into Reality Engineers do practical design and production work using the principles of science and mathematics to come up with economically viable solutions to technical problems. . In shorter words, they design and build needed tools, products, and systems. What is mechanical engineering ? Mechanical engineering is the most diverse and exciting of all the engineering disciplines. . It is specifically concerned with the design, development, installation, operation and maintenance of just about anything that has moveable parts. . If an object is man-made, mechanical engineering skills will have been involved at some stage during its development and manufacture. Role of Mechanical Engineers We Make Machines… and More ! Mechanical engineers are professionals devoted to employing the principles of motion, forces, and energy. Mechanical engineers research, develop, design, manufacture test tools, engines, machines, and other mechanical devices. For society this means that mechanical engineers are using their wide range of skills to think of ways to improve the way we live. . 2、Top 10 Benefits The top 10 rewards and opportunities that an engineering career offers. #1 Job Satisfaction It is important to find a career that provides you with enjoyment and satisfaction. . After all, you might spend 40 or so years working eight hours or more a day, five days a week, 50 weeks a year.. Do you want to dislike every minute of that time, or would you rather do something that you enjoy? . #2 Varied Opportunities An engineering degree offers a wide range of career possibilities. If you are imaginative and creative, design engineering may be for you. If you like laboratories and conducting experiments, you might consider test engineering. If you like to organize and expedite projects, look into being a development engineer. If you are persuasive and like working with people, consider a career in sales or field service engineering. The analytical skills and technological expertise you develop as an engineering student can also be put to use in many other fields.
For example, you could combine engineering and business skills in a career as a technical manager or a salesperson for a high-tech company #3 Challenging Work If you like challenges, engineering could be for you. In the engineering work world, there is no shortage of challenging problems When you get into the engineering work world, virtually all problems will be open-ended. There will be no single answer, no answer in the back of the book, no professor to tell you that you are right or wrong You will be required to devise a solution and persuade others that your solution is the best one #4 Intellectual Development An engineering education will"exercise" your brain, developing your ability to think logically and to These are skills that will be valuable throughout your life- and not only when you are solving engineering problems For example, your problem-solving skills can help you undertake tasks such as planning a vacation, finding a job, organizing a fund-raiser, purchasing a house, or writing a book #5 Social Impact Just about everything that engineers do benefits society. For example, engineers develop transportation systems that help people and products move about As an engineer, you can also work on beneficial projects, such as cleaning up the environment, finding new sources of energy and increasing the standard of living in underdeveloped countries #6 Financial Security While financial security should not be your only reason for choosing a career in engineering, if you decide to become an engineer, you will be well paid Pr stige Engineers play a primary role in sustaining our nations international competitiveness, maintaining our standard of living, ensuring a strong national security, and protecting public safety As a member of such a respected profession, you will receive a high amount of prestige #8 Professional Environment As an engineer, you will work in a professional environment in which you will be treated with respect and have a certain amount of freedom in choosing your work. You will be also be in a position to influence what happens at your company In most cases, you will receive adequate work space and the tools you need to do your work, including the latest computer hardware and software. You will probably also receive the secretarial and technical support staff you need to get your work done You will learn from experienced engineers in your organization and will be offered seminars and short courses to increase your knowledge #9 Technological and Scientific Discovery An engineering education can help you understand how things in the world work Furthermore, an understanding of technology will also provide you with a better understanding of lany issues facing our society For example: Should we have stopped building nuclear reactors? What will we use for energy when oil runs out?
For example, you could combine engineering and business skills in a career as a technical manager or a salesperson for a high-tech company. #3 Challenging Work If you like challenges, engineering could be for you. In the engineering work world, there is no shortage of challenging problems. . When you get into the engineering work world, virtually all problems will be open-ended. There will be no single answer, no answer in the back of the book, no professor to tell you that you are right or wrong. . You will be required to devise a solution and persuade others that your solution is the best one. #4 Intellectual Development An engineering education will “exercise” your brain, developing your ability to think logically and to solve problems. These are skills that will be valuable throughout your life — and not only when you are solving engineering problems. . For example, your problem-solving skills can help you undertake tasks such as planning a vacation, finding a job, organizing a fund-raiser, purchasing a house, or writing a book. #5 Social Impact Just about everything that engineers do benefits society. For example, engineers develop transportation systems that help people and products move about easily. . As an engineer, you can also work on beneficial projects, such as cleaning up the environment, finding new sources of energy and increasing the standard of living in underdeveloped countries. #6 Financial Security While financial security should not be your only reason for choosing a career in engineering, if you decide to become an engineer, you will be well paid. #7 Prestige Engineers play a primary role in sustaining our nation’s international competitiveness, maintaining our standard of living, ensuring a strong national security, and protecting public safety. As a member of such a respected profession, you will receive a high amount of prestige. #8 Professional Environment As an engineer, you will work in a professional environment in which you will be treated with respect and have a certain amount of freedom in choosing your work. You will be also be in a position to influence what happens at your company. In most cases, you will receive adequate work space and the tools you need to do your work, including the latest computer hardware and software. You will probably also receive the secretarial and technical support staff you need to get your work done. . You will learn from experienced engineers in your organization and will be offered seminars and short courses to increase your knowledge. #9 Technological and Scientific Discovery An engineering education can help you understand how things in the world work. Furthermore, an understanding of technology will also provide you with a better understanding of many issues facing our society. For example: Should we have stopped building nuclear reactors? What will we use for energy when oil runs out?
#10 Creative Thinking Because we are in a time of rapid social and technological changes the need for engineers to think creatively is greater now than ever before Only through creativity can we cope with and adapt to these changes. If you like to question xplore, invent, discover, and create, then engineering could be the ideal profession for you! 3、Top10 achievements The top 10 Great Achievements of 20th Century Mechanical Engineering 1.The Automobile Henry Ford freed common people from the limitations of geography, creating social mobility on a scale previously unknown 2. Apollo Moon Landing From early test rockets to sophisticated satellites, the human expansion into space is perhaps the most amazing engineering feat 3. Power Generati。n From street lights to supercomputers, electric power makes our lives safer, healthier, and more convenien 4. Agricultural Mechanization The machinery of farms-tractors, cultivators, combines, and hundreds of others-dramatically increased farm efficiency and productivity 5. The airplane Modern air travel transports goods and people quickly around the globe, facilitating our personal cultural and commercial interaction 6.Integrated circuit From vacuum tubes to transistors to integrated circuits, engineers have made electronics smaller more powerful, and more efficient 7. Air conditioning and refrigeration No longer dependent on the weather for work or play, humans truly made the environment adapt to their need 8. CAD/CAM and CAE Technology Computers and imaging technologies are used to design and test a variety of products 9. Bioengineering Artificial organs, replacement joints, and biomaterials are but a few of the engineered products that improve the quality of life for millions 10. Codes and standards Standards for product design and performance promote product quality, safety, and interchangeability of components 4、 New directions in Me Micro/Nano Technologies Micro-Electro-Mechanical Systems(MEMS)is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology MEMS (Micro-electro-mechanical systems) requires the solution of many mechanical engineering problems on the micro scale Fluid control: Microvalve/pump, Microsensor; Computer: Magnetic head, Printer head, Laser scanner, Micro-mechanical memory
#10 Creative Thinking Because we are in a time of rapid social and technological changes, the need for engineers to think creatively is greater now than ever before. . Only through creativity can we cope with and adapt to these changes. If you like to question, explore, invent, discover, and create, then engineering could be the ideal profession for you! 3、Top 10 achievements The top 10 Great Achievements of 20th Century Mechanical Engineering. 1.The Automobile .Henry Ford freed common people from the limitations of geography, creating social mobility on a scale previously unknown. 2.Apollo Moon Landing From early test rockets to sophisticated satellites, the human expansion into space is perhaps the most amazing engineering feat. 3.Power Generation From street lights to supercomputers, electric power makes our lives safer, healthier, and more convenient. 4.Agricultural Mechanization The machinery of farms - tractors, cultivators, combines, and hundreds of others - dramatically increased farm efficiency and productivity. 5.The airplane Modern air travel transports goods and people quickly around the globe, facilitating our personal, cultural, and commercial interaction. 6.Integrated circuit From vacuum tubes to transistors to integrated circuits, engineers have made electronics smaller, more powerful, and more efficient. 7.Air conditioning and refrigeration No longer dependent on the weather for work or play, humans truly made the environment adapt to their needs. 8.CAD/ CAM and CAE Technology Computers and imaging technologies are used to design and test a variety of products. 9.Bioengineering Artificial organs, replacement joints, and biomaterials are but a few of the engineered products that improve the quality of life for millions. 10.Codes and standards Standards for product design and performance promote product quality, safety, and interchangeability of components. 4、New directions in ME Micro/Nano Technologies Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through microfabrication technology. MEMS (Micro-electro-mechanical systems) requires the solution of many mechanical engineering problems on the micro scale. . Fluid control : Microvalve/pump, Microsensor ; Computer : Magnetic head, Printer head, Laser scanner, Micro-mechanical memory ;
Robot: Microrobots, Micro-teleoperator, Mobile sensor. Figure shows Nickel micromotor and gear train formed using the LIGA process at the University of Wisconsin, Such structures combine extreme precision with high aspect ratios, can be driven magnetically, and provide one example of MEMS. The rotor diameter here is 150 mm Magnetic micromotors have been driven at rates exceeding 50,000 rpm Cellular and molecular biomechanics In recent years cellular and molecular bio-mechanics have gained in importance, and problems of biomechanics at that scale have begun to emerge These problems, again, necessitate expansion and further development in the basic continuum mechanics theories and models that have long been the mainstay of ME Information Technology T has influenced ME in many significant ways Computational methods in mechanics are becoming increasingly important, e.g., finite element lods(FEM) The availability of distributed information through networks reinforces the emphasis on collaborative systems design and analysis rather than a more isolated focus on the various components Energy and Environmental Issues The focus in environmental engineering has shifted from remediation(e.g, wastewater treatment)to design of environmentally "friendly products manufactured in an environmentally conscious manner Key principles with which mechanical engineers must deal are design for the environment, lifecycle design, and sustainable development 5. Communicating as a ME In both academia and industry, mechanical engineers need to speak and write their ideas 5.1 Types of communication Lab Reports Lab work is an important part of every engineer's training. Lab Reports are factual presentations of test o experiment results completed in a lab or simulation. Typically, Lab reports discuss procedures as well as describe the details of a test or experiment Poster sessions As an engineer, you'll participate in Poster Sessions during conferences and seminars. A Poster Session allows you to display and discuss your work on a project or the results of your research. It combines text and graphics to make a visually-pleasing presentation. As viewers walk by, your poster should quickly and efficiently communicate your research Proposals Engineers write Proposals to present a topic to be researched or to suggest a plan of action In academia, engineers write proposals to receive funding for their research or even to initiate a project As an engineer, you may determine that a problem exists, and therefore, propose solutions to an organization. In this case, you must first convince the agency that the problem exists before proposing your solutions Technical Reports Technical reports present facts and conclusions about your designs and other projects. Typically, a technical report includes research about technical concepts as well as graphical depictions of designs and A technical report follows a strict organization. This way, when other engineers read what you write, they can quickly locate the information that interests them the most
Robot : Microrobots, Micro-teleoperator, Mobile sensor. Figure shows Nickel micromotor and gear train formed using the LIGA process at the University of Wisconsin ,Such structures combine extreme precision with high aspect ratios, can be driven magnetically, and provide one example of MEMS. The rotor diameter here is 150 mm. Magnetic micromotors have been driven at rates exceeding 50,000 rpm. Cellular and Molecular Biomechanics In recent years cellular and molecular bio-mechanics have gained in importance, and problems of biomechanics at that scale have begun to emerge. These problems, again, necessitate expansion and further development in the basic continuum mechanics theories and models that have long been the mainstay of ME. Information Technology IT has influenced ME in many significant ways. Computational methods in mechanics are becoming increasingly important, e.g., finite element methods (FEM). The availability of distributed information through networks reinforces the emphasis on collaborative systems design and analysis rather than a more isolated focus on the various components. Energy and Environmental Issues The focus in environmental engineering has shifted from remediation (e.g., wastewater treatment) to design of environmentally “friendly” products manufactured in an environmentally conscious manner. . Key principles with which mechanical engineers must deal are design for the environment, lifecycle design, and sustainable development. . 5. Communicating as a ME In both academia and industry, mechanical engineers need to speak and write their ideas. 5.1 Types of communication Lab Reports Lab work is an important part of every engineer's training. Lab Reports are factual presentations of test or experiment results completed in a lab or simulation. Typically, Lab Reports discuss procedures as well as describe the details of a test or experiment. Poster Sessions As an engineer, you'll participate in Poster Sessions during conferences and seminars. A Poster Session allows you to display and discuss your work on a project or the results of your research. . It combines text and graphics to make a visually-pleasing presentation. As viewers walk by, your poster should quickly and efficiently communicate your research. Proposals Engineers write Proposals to present a topic to be researched or to suggest a plan of action. . In academia, engineers write proposals to receive funding for their research or even to initiate a project. As an engineer, you may determine that a problem exists, and therefore, propose solutions to an organization. In this case, you must first convince the agency that the problem exists before proposing your solutions. Technical Reports Technical reports present facts and conclusions about your designs and other projects. Typically, a technical report includes research about technical concepts as well as graphical depictions of designs and data. . A technical report follows a strict organization. This way, when other engineers read what you write, they can quickly locate the information that interests them the most.
5.2 Communication Conventions Headings Subheadings They are good organizational techniques, and they also help readers locate information. This way, a reader interested in the necessary materials could quickly find this information without reading the whole repor Lists are effective ways to present information. Lists are especially useful when you have to convey steps, phases, years, procedures, or decisions When creating a list, consider writing phrases, fragments or even questions and answers. By avoiding full sentences in a list, your information is concise and more likely to engage your readers Passive voice Usually, the passive voice should be used in writing 1. I used the electric identifier to solve the problem. active voice 2. The electric identifier was used to solve the problem. passive voice Tersene engthy sentences and long paragraphs are signs that your writing is not terse. The reason why terseness is necessary to good engineering writing is because it helps your readers understand information Good writing is descriptive, but it also gets to the point as quickly as possible. The information you present should always be relevant to your topic, as well as to your audience 5.3 Advice from engineers Present Information Logically Many engineering professors note that much of the writing they read from students often doesn't have a logical flow. By this, they mean that the writing doesn,'t present ideas in an order that makes sense You should also make sure that your entire document or presentation presents information logically. For instance, don't include conclusions or results in either the procedure section or the introduction. Format your documents Whenever you produce a document, you should always consider how you've organized your thoughts and how you can make this known to the reader You should use a consistent style(according to the style guidelines in your discipline). This includes margin sizes, line spacing, and even the title page you attach to the front of your document Know Your Purpose and audience By determining who your audience is and what your purpose is, you can then gather specific information nstead of including everything that you might find on a particular topic This way, you don't have to worry about presenting information that may bore or confuse a particular audience 6. Summary Mechanical engineers are problem solvers who are employed in a wide variety of areas. They work in a team environment, often with professionals from other disciplines Mechanical engineers work in many industries, often developing everyday products. They are also employed in government, doing research, and in academia, teaching the next generation of engineers. Thus, mechanical engineers play an important role in society
5.2 Communication Conventions Headings & Subheadings They are good organizational techniques, and they also help readers locate information. This way, a reader interested in the necessary materials could quickly find this information without reading the whole report. Lists Lists are effective ways to present information. Lists are especially useful when you have to convey steps, phases, years, procedures, or decisions. When creating a list, consider writing phrases, fragments or even questions and answers. By avoiding full sentences in a list, your information is concise and more likely to engage your readers. Passive Voice Usually, the passive voice should be used in writing. 1. I used the electric identifier to solve the problem. ( active voice ) . 2. The electric identifier was used to solve the problem.( passive voice ) . Terseness Lengthy sentences and long paragraphs are signs that your writing is not terse. The reason why terseness is necessary to good engineering writing is because it helps your readers understand information quicker. . Good writing is descriptive, but it also gets to the point as quickly as possible. The information you present should always be relevant to your topic, as well as to your audience. . 5.3 Advice from engineers Present Information Logically Many engineering professors note that much of the writing they read from students often doesn't have a "logical flow." By this, they mean that the writing doesn't present ideas in an order that makes sense. You should also make sure that your entire document or presentation presents information logically. For instance, don't include conclusions or results in either the procedure section or the introduction. Format Your Documents Whenever you produce a document, you should always consider how you've organized your thoughts and how you can make this known to the reader. You should use a consistent style (according to the style guidelines in your discipline). This includes margin sizes, line spacing, and even the title page you attach to the front of your document. . Know Your Purpose and Audience By determining who your audience is and what your purpose is, you can then gather specific information instead of including everything that you might find on a particular topic. . This way, you don't have to worry about presenting information that may bore or confuse a particular audience. 6. Summary - Mechanical engineers are problem solvers who are employed in a wide variety of areas. They work in a team environment, often with professionals from other disciplines. . - Mechanical engineers work in many industries, often developing everyday products. They are also employed in government, doing research , and in academia, teaching the next generation of engineers. Thus, mechanical engineers play an important role in society
Statics, Dynamics and Mechanical Engineering 1、 Introduction Mechanics: Science which describes and predicts the conditions of rest or motion of bodies under the action of forces The field of Classical Mechanics can be divided into three categories 1)Mechanics of Rigid Bodies 2) Mechanics of Deformable Bod 3)Mechanics of Fluids Rigid-body mechanics General mechanics Statics deals with bodies that are in equilibrium with applied forces. I Such bodies are either at rest or moving at a constant velocity Dynamics deals with the relation between forces and the motion of bodies. Bodies are accelerating. I P Rigid-body mechanics is based on the Newtons laws ofmotion These laws were postulated for a particle, which has a mass, but no size or shap Newton's laws may be extended to rigid bodies by considering the rigid body to be made up of a large numbers of particles whose relative positions from each other do not change Newton's Laws of motion Ist law. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. 2nd law. If the resultant force acting on a particle is not zero, the particle will experience an acceleration proportional to the magnitude of the force and in the direction of this resultant force. 3rd law. The mutual forces of action and reaction between two particles are equal in magnitude, opposite in direction, and collinear 2.1 Vectors g Scalar: Any quantity possessing magnitude(size)only, such as mass, volume, temperature g Vector: Any quantity possessing both magnitude and direction, such as force, velocity, momentum The calculation of a vector must be in a reference frame. A scalar is independent of reference frames Given two vectors, the vectors will only be equal if both the magnitude and direction of both vectors In Cartesian coordinate system, an arbitrary vector can be written in terms of unit vectors as Addition of two vectors Subtraction of Two vectors Inner product of Two vectors Vector Product of two vectors 22 Forces Force is a vector quantity, a force is completely described by: 1. Magnitude2 Direction3 Point of External force: Forces caused by other bodies acting on the rigid body being studied. EX--weight pushing, pulling Internal force Those forces that keep the rigid body together Force in 3D Aforce F in three-dimensional space can be resolved into components using the unit vectors
Statics, Dynamics and Mechanical Engineering 1、Introduction Mechanics: Science which describes and predicts the conditions of rest or motion of bodies under the action of forces. The field of Classical Mechanics can be divided into three categories : . 1) Mechanics of Rigid Bodies 2) Mechanics of Deformable Bodies 3) Mechanics of Fluids Rigid-body mechanics ( General mechanics ) Statics deals with bodies that are in equilibrium with applied forces. [ Such bodies are either at rest or moving at a constant velocity. ] . Dynamics deals with the relation between forces and the motion of bodies. [ Bodies are accelerating. ] Notes ➢Rigid-body mechanics is based on the Newton’s laws ofmotion. ➢ These laws were postulated for a particle, which has a mass, but no size or shape. . ➢ Newton’s laws may be extended to rigid bodies by considering the rigid body to be made up of a large numbers of particles whose relative positions from each other do not change. Newton’s Laws of Motion 1st law. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. 2nd law. If the resultant force acting on a particle is not zero, the particle will experience an acceleration proportional to the magnitude of the force and in the direction of this resultant force. 3rd law. The mutual forces of action and reaction between two particles are equal in magnitude, opposite in direction, and collinear. 2.1 Vectors ❖ Scalar : Any quantity possessing magnitude (size) only, such as mass, volume, temperature. ❖ Vector : Any quantity possessing both magnitude and direction, such as force, velocity, momentum. The calculation of a vector must be in a reference frame. A scalar is independent of reference frames. Given two vectors, the vectors will only be equal if both the magnitude and direction of both vectors are equal. In Cartesian coordinate system, an arbitrary vector can be written in terms of unit vectors as Addition of Two Vectors Subtraction of Two Vectors Inner Product of Two Vectors Vector Product of Two Vectors 2.2 Forces Force is a vector quantity, a force is completely described by:1.Magnitude2.Direction3.Point of Application External force : Forces caused by other bodies acting on the rigid body being studied. ( Ex.-- weight, pushing, pulling. ) Internal force : Those forces that keep the rigid body together. Force in 3D A force F in three-dimensional space can be resolved into components using the unit vectors :
The vectors i, j, k are unit vectors along the x, y and z axes respectively 2.3 The moment of force F about point O is defined as the vector product where r is the position vector drawn from point O to the point of application of the force F The right-hand rule is used to indicate a positive moment. torque 24c。 uples A couple is formed by 2 forces F and-F that have equal magnitudes, parallel lines of action and opposite The moment of a couple is a vector M perpendicular to the plane of the couple and equal in magnitude to the product Fd. Notes Acouple will not cause translation only rotation The moment of a couple is independent of the point about which it is computed Two couples having the same moment M are equivalent, They have the same effect on a given rigid body The direction of a couple is given by the right-hand rule. Therefore, a positive couple generates rotation in a counterclockwise sense 2.5 Equilibrium of a Rigid Body Conditions for rigid-body equilibrium where Forces are"external forces"( body force, applied force, support reaction Moment may be taken about any center of rotation"o 2.6 Free Body Diagrams( FBD) Three steps in drawing a free body diagram Isolate the body, remove all supports and connectors 2. Identify all external forces acting on the body. 3. Make a sketch of the body, showing all forces acting on 2.7 Solving a Statics Problem STEPS 1. Draw a free body diagram 2. Choose a reference frame. Orient the axes 3. Choose a convenient point to calculate moments around 4. Apply the equilibrium equations and solve for the unknowns 2.8 Frictional Forces In problems involving the contact of two bodies, if the contact is not smooth, a reaction will occur along the line of contact. This reaction is a force of resistance called the friction. Frictional forces inhibit or prevent slipping Provided that there is no slipping at the contact surface and that the body is not accelerating, experimental studies have shown that the frictional force is related to the normal contact force by the equation: F=us M Where F is the static frictional force and N is the normal contact force. The constant us is called the If the body is accelerating, then the frictional force has a value less than the static value. This frictional force, F, called the kinetic frictional force and is related to the normal force as F=uk where uk is the coefficient of kinetic friction. Values of uk are as much as 25% smaller than values for u 3. Dynamics Dynamics Kinematics Kinetics 1). Kinematics, branch of dynamics concerned with describing the state of motion of bodies without regard to the causes of the motion. displacement, velocity, acceleration, and time
The vectors i, j, k are unit vectors along the x, y and z axes respectively. . 2.3 Moments The moment of force F about point O is defined as the vector product : where r is the position vector drawn from point O to the point of application of the force F. . The right-hand rule is used to indicate a positive moment. ( torque ) 2.4 Couples A couple is formed by 2 forces F and -F that have equal magnitudes, parallel lines of action and opposite direction. The moment of a couple is a vector M perpendicular to the plane of the couple and equal in magnitude to the product Fd. Notes @ A couple will not cause translation only rotation. @ The moment of a couple is independent of the point about which it is computed. @ Two couples having the same moment M are equivalent. They have the same effect on a given rigid body. The direction of a couple is given by the right-hand rule. Therefore, a positive couple generates rotation in a counterclockwise sense. 2.5 Equilibrium of a Rigid Body Conditions for rigid-body equilibrium : where: • Forces are “external forces” ( body force, applied force, support reaction ) • Moment may be taken about any center of rotation “o” 2.6 Free Body Diagrams ( FBD ) Three steps in drawing a free body diagram: 1. Isolate the body, remove all supports and connectors. 2. Identify all external forces acting on the body. 3. Make a sketch of the body, showing all forces acting on it. 2.7 Solving a Statics Problem STEPS: 1. Draw a free body diagram. 2. Choose a reference frame. Orient the axes. 3. Choose a convenient point to calculate moments around. 4.Apply the equilibrium equations and solve for the unknowns 2.8 Frictional Forces In problems involving the contact of two bodies, if the contact is not smooth, a reaction will occur along the line of contact. This reaction is a force of resistance called the friction. Frictional forces inhibit or prevent slipping. Provided that there is no slipping at the contact surface and that the body is not accelerating, experimental studies have shown that the frictional force is related to the normal contact force by the equation : F = µs N Where F is the static frictional force and N is the normal contact force. The constant µs is called the coefficient of static friction. If the body is accelerating, then the frictional force has a value less than the static value. This frictional force, F, is called the kinetic frictional force and is related to the normal force as F = µk N where μk is the coefficient of kinetic friction. Values of μk are as much as 25% smaller than values for μ s . 3. Dynamics Dynamics = Kinematics + Kinetics 1). Kinematics, branch of dynamics concerned with describing the state of motion of bodies without regard to the causes of the motion. [ displacement, velocity, acceleration, and time ]
2). Kinetics, branch of dynamics concerned with causes of motion and the action of forces work, power, energy, impulse,.] Direct dynamics: Calculation of kinematics from forces applied to bodies Inverse dynamics: Calculation of forces and moments from kinematics of bodies and their inertial properties Applications: Analysis of cams, gears, shafts, linkages, connecting rods, etc 3.1 Kinematics Types of rigid-body motion Translation( 3 degrees of freedom) Rotation about a fixed axis (1 DOF)(angular velocity w, angular acceleration a General plane motion (3 DOF the sum of a translation and a rotation Motion about a fixed point(3 DOF) General motion (6 DOF) Equations of motion for rigid bodies Where m is the mass of the rigid body, a is the acceleration of the body s center of mass, / is called the mass moment of inertia(in kg m2), and a is the angular acceleration of the center of mass(in rad/s 2 3.3 Solving a Dynamics Problem Free body diagrams Equations of motion The acceleration and angular acceleration must be indicated on the diagram. 4、 Summary Rigid-body mechanics, which includes statics and dynamics, is a branch of science that deals with forces and motion of bodies that do not deform under the applied loads In a free-body diagram, the body under considera-tion is isolated from its surrounding, and loads acting on the body are shown. The direction and magnitudes of the loads must be properly indicated or the analysis will fa Solid Mechanics and Mechanical Engineering Objectives After learning this chapter you should be able to do the following Differentiate between the different types of basic loading conditions . Understand the basic approach of the Finite Element Method( FEM) 1 Introduction During the analysis of an engineering design, a mechanical engineer is often faced with predicting the deformation of a body In some cases, the inverse problem is solved. That is, the maximum amount of desired deformation is known and the load that will produce the deformation is desired Solid Mechanics: Structural Mechanics, Mechanics of Materials, Elastic Mechanics. Plastic Mechanics
2). Kinetics, branch of dynamics concerned with causes of motion and the action of forces. . [ work, power, energy, impulse, …] Direct dynamics:Calculation of kinematics from forces applied to bodies. Inverse dynamics:Calculation of forces and moments from kinematics of bodies and their inertial properties. Applications : Analysis of cams, gears, shafts, linkages, connecting rods, etc. 3.1 Kinematics Types of rigid-body motion : Translation (3 degrees of freedom) Rotation about a fixed axis (1 DOF) (angular velocity ω, angular acceleration α ) General plane motion(3 DOF)( the sum of a translation and a rotation ) Motion about a fixed point (3 DOF) General motion (6 DOF) Equations of motion for rigid bodies : Where m is the mass of the rigid body, a is the acceleration of the body’s center of mass, I is called the mass moment of inertia (in kg·m2), and α is the angular acceleration of the center of mass (in rad/s2). 3.3 Solving a Dynamics Problem Free body diagrams Equations of motion The acceleration and angular acceleration must be indicated on the diagram. 4、Summary Rigid-body mechanics, which includes statics and dynamics, is a branch of science that deals with forces and motion of bodies that do not deform under the applied loads. In a free-body diagram, the body under considera- tion is isolated from its surrounding, and loads acting on the body are shown. The direction and magnitudes of the loads must be properly indicated or the analysis will fail. Solid Mechanics and Mechanical Engineering Objectives After learning this chapter, you should be able to do the following : ❖ Differentiate between the different types of basic loading conditions. . ❖ Understand the basic approach of the Finite Element Method(FEM). 1. Introduction During the analysis of an engineering design, a mechanical engineer is often faced with predicting the deformation of a body. . In some cases, the inverse problem is solved. That is, the maximum amount of desired deformation is known and the load that will produce the deformation is desired. Solid Mechanics : Structural Mechanics、Mechanics of Materials、Elastic Mechanics、Plastic Mechanics
2. Stress and strain Normal Stress, often symbolized by the greek letter sigma, is defined as the force perpendicular to the cross ectional area divided by the cross sectional area. (axial Axial Strain, a non-dimensional parameter, is defined as the ratio of the deformation in length to the original length Strain is often represented by the Greek symbol epsilon( Application Suppose the force is perpendicular to the longitudinal axis. The stress will be a Shear Stress, defined as force parallel to an area divided by the area. Just as an axial stress results in an axial strain, so does shear stress produce a Shear Strain(n) Application 2----Shearing Force Let's consider a shaft, to which an external torgue is applied(such as in power transmission). The shaft is said to be in torsion. The effect of torsion is to create an angular displacement of one end of the shaft with respect to the other. For a shaft of circular cross section, the relationship between the shear stress and torque is where J is the polar moment of inertia Application 3----Transmission Shaft Notes In general, more than one type of stress may be active in a solid body, due to combined loading conditions. (tension, compression, shear, torsion, etc. )When faced with an engineering problem, an engineer must recognize if more than state of stress exists. Because stresses are vector quantities, care must be taken when adding the terms together Application Transmission system of machine tools Notes The simple loading cases considered in this chapter form the basics of the study of strength of material Method is often used to solve problems involving complicated geometries or loading conditions for structural ar 3. Poisson Effect When a tensile load is applied to a uniform beam the increase in the length of the beam is accompanied by decrease in the lateral dimension of the beam The decrease or the increase in the lateral dimension is due to a lateral strain, which is proportional to the strain along the axial direction The ratio of the lateral strain to the axial strain is related to the poisson ratio named after the mathematician who calculated the ratio by molecular theory The minus sign in Equation is needed in order to keep track of the sign in the strain. For example, because tension corresponds to a decrease in the lateral direction, the lateral strain is negative 4 Hookes law Hooke's Law says that the stretch of a spring is directly proportional to the applied force. Engineers say "Stress is proportional to stra This law is formulated in terms of the stress and strain and may be written as where E is a material constant known as Young's modulus Example 1
2. Stress and Strain . Normal Stress, often symbolized by the Greek letter sigma, is defined as the force perpendicular to the cross sectional area divided by the cross sectional area. (axial stress) . . Axial Strain, a non-dimensional parameter, is defined as the ratio of the deformation in length to the original length. Strain is often represented by the Greek symbol epsilon( ). Application 1——（Tension & Compression） Suppose the force is perpendicular to the longitudinal axis. The stress will be a Shear Stress, defined as force parallel to an area divided by the area..Just as an axial stress results in an axial strain, so does shear stress produce a Shear Strain (γ). Application 2——Shearing Force Let’s consider a shaft, to which an external torque is applied (such as in power transmission). The shaft is said to be in torsion. The effect of torsion is to create an angular displacement of one end of the shaft with respect to the other. For a shaft of circular cross section, the relationship between the shear stress and torque is where J is the polar moment of inertia. Application 3——Transmission Shaft Notes In general, more than one type of stress may be active in a solid body, due to combined loading conditions.(tension, compression, shear, torsion, etc.) When faced with an engineering problem, an engineer must recognize if more than state of stress exists.. Because stresses are vector quantities, care must be taken when adding the terms together. Application 4——Transmission system of machine tools Notes The simple loading cases considered in this chapter form the basics of the study of strength of materials. .. The Finite Element Method is often used to solve problems involving complicated geometries or loading conditions for structural analysis. 3. Poisson Effect When a tensile load is applied to a uniform beam, the increase in the length of the beam is accompanied by a decrease in the lateral dimension of the beam. . The decrease or the increase in the lateral dimension is due to a lateral strain, which is proportional to the strain along the axial direction. The ratio of the lateral strain to the axial strain is related to the Poisson ratio, named after the mathematician who calculated the ratio by molecular theory. The minus sign in Equation is needed in order to keep track of the sign in the strain. For example, because tension corresponds to a decrease in the lateral direction, the lateral strain is negative. 4. Hooke’s Law Hooke's Law says that the stretch of a spring is directly proportional to the applied force. Engineers say "Stress is proportional to strain". This law is formulated in terms of the stress and strain and may be written as : where E is a material constant known as Young’s modulus. Example 1
Suppose that a 4-inch-diameter round bar is extended with a 50.000-lb axial load. The bar has an initial length of 5 feet and extends 0.006 inches. What is the Youngs modulus for the material from which the bar is Solution We can obtain the Youngs modulus by using Hooke's law, and Equation (1)and Equation(2) The stress in the bar is Thus the Young's modulus for this material Stress concentration When an elastic body with a local geometrical irregularity is stressed, there usually is a localized variation in the stress state in the immediate neighborhood of the irregularity The maximum stress levels at the irregularity may be several times larger than the nominal stress levels in the bulk of the body. This increase in stress caused by the irregularity in geometry is called a stress concentration Stress Concentration Factor Where the stress concentration can not be avoided by a change in design, it is important to base the design on the local value of the stress rather than on an average value. The usual procedure in design is to obtain the local value of the stress by use of a stress concentration factor ss in the presence of a geometric irregularity or discontinuity, a nom, nominal stress which would exist at the point if the irregularity were not there Typical kt Curve 6. Fatigue Loads or deformations which will not cause fracture in a single application can result in fracture when applied repeatedly Fracture may occur after a few cycles, or after millions of cycles This process of fracture under repeated loading is called fatigue. Fatigue is one of the three common causes of mechanical failure. the others being wear and corrosion. Consider a situation in which the stress at a point in a body varies with time Experiments show that the alternating stress o a is the most important factor in determining the number of cycles of load a material can withstand before fracture, while the mean stress level a m is less important Fatigue Curve Notes It is customary to designate the stress which can be withstood for some specified number of cycles as the fatigue strength of the material Fatigue cracks are most likely to form and grow from locations where holes or sharp corners cause stress In designing parts to withstand repeated stresses, it is important to avoid stress concentrations. Keyways, oil holes, and screw threads are potential sources of trouble and require special care in design in order to prevent fatigue failures 7. Finite Element Method ( FEM) The FEM is a numerical analysis technique for obtaining approximate solutions to engineering and design
Suppose that a 4-inch-diameter round bar is extended with a 50.000-lb axial load. The bar has an initial length of 5 feet and extends 0.006 inches. What is the Young’s modulus for the material from which the bar is made? . Solution We can obtain the Young’s modulus by using Hooke’s law, and Equation (1) and Equation (2). The stress in the bar is The strain is Thus, the Young’s modulus for this material is 5. Stress Concentration When an elastic body with a local geometrical irregularity is stressed, there usually is a localized variation in the stress state in the immediate neighborhood of the irregularity. The maximum stress levels at the irregularity may be several times larger than the nominal stress levels in the bulk of the body. This increase in stress caused by the irregularity in geometry is called a stress concentration. Stress Concentration Factor Where the stress concentration can not be avoided by a change in design, it is important to base the design on the local value of the stress rather than on an average value. The usual procedure in design is to obtain the local value of the stress by use of a stress concentration factor. σmax , maximum stress in the presence of a geometric irregularity or discontinuity, σnom, nominal stress which would exist at the point if the irregularity were not there. Typical Kt Curve 6. Fatigue Loads or deformations which will not cause fracture in a single application can result in fracture when applied repeatedly. Fracture may occur after a few cycles, or after millions of cycles. . This process of fracture under repeated loading is called fatigue. Fatigue is one of the three common causes of mechanical failure, the others being wear and corrosion. Consider a situation in which the stress at a point in a body varies with time. Experiments show that the alternating stress σa is the most important factor in determining the number of cycles of load a material can withstand before fracture, while the mean stress level σm is less important. Fatigue Curve Notes It is customary to designate the stress which can be withstood for some specified number of cycles as the fatigue strength of the material. . Fatigue cracks are most likely to form and grow from locations where holes or sharp corners cause stress concentrations. . In designing parts to withstand repeated stresses, it is important to avoid stress concentrations. Keyways, oil holes, and screw threads are potential sources of trouble and require special care in design in order to prevent fatigue failures. 7. Finite Element Method (FEM) The FEM is a numerical analysis technique for obtaining approximate solutions to engineering and design problems
