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UNIT2.6-Mechanisms: Eng Notes EP@BHS-TOPIC 2: Energy, UNIT2.6: Mechanisms Page 1 UNIT 2.6 MECHANISMS: Concepts Addressed in Lesson: 1. Most mechanisms are composed of simple machines, interlocking gears, chain driven sprockets, and belt driven pulleys. 2. Mechanisms are used to transmit energy through a system by manipulating force, speed, and direction. 3. Mechanical advantages mathematically represent the ratio of the force output to the force input for mechanisms. Performance Objectives Addressed in Lesson: It is expected that students will: o Measure forces, speeds, and distances as related to the operation of mechanisms. o Distinguish between the six simple machines, their attributes, and components. o Calculate ideal mechanical advantages, gear ratios, and drive ratios for mechanisms. o Design, create, and test gear, belt-pulley, and/or chain-sprocket systems. o Calculate work, power, torque, and efficiency for mechanical systems. Assessment: Explanation • Students will explain the difference between engineering and engineering technology. • Students will explain the relationship between work and power in a mechanical system. • Students will explain the processes of calculating mechanical advantage. Interpretation • Students will make journal entries reflecting on their learning experiences. • Students will explain the importance and relevance of simple machines in everyday life. Application • Students will apply their knowledge of simple machines and calculate mechanical advantage of objects within the lab environment. • Students will apply their knowledge of system efficiency to calculate efficiency of a mechanical system. • Students will apply their knowledge of gear, sprocket, and pulley systems to calculate speed, distance, rotational direction, and mechanical advantage. Perspective • Students will select an engineering or engineering technology field of interest and prepare an interview with a professional within the field of interest. • Students will identify and discuss the role and impact of simple machines, compound machines, and gears, pulleys, and sprockets throughout the development of civilizations. Self-knowledge • Students will be required to reflect on their work in their work-guides • Students will conduct formal periodic self-assessments of course knowledge and content. Essential Questions 1. Why is it important to begin considering career paths during high school? 2. What career opportunities are available to match your specific interests? 3. What are some current applications of simple machines, gears, pulleys, and sprockets? 4. What are some strategies that can be used to make everyday mechanisms more efficient? 5. What are the trade-offs of mechanical advantage related to design? 6. Why must efficiency be calculated and understood during the design process? Date Completed: Preliminary Final Grader Name: Initials: Grade: Grade: Initials: EP@BHS-TOPIC 2: Energy, UNIT2.6: Mechanisms Page 2 A. Simple Machines We will begin this lesson by consulting the Merriam-Webster Dictionary to obtain a specific definition for the field of “mechanical engineering”: It is apparent from this definition, that mechanical engineers must understand the fundamentals of tools and machinery, so let’s now use the same dictionary to explore the various definitions of “machine”: For our current purposes, the most important definition listed under “machine” is 1.e(1): an assemblage of parts that transmit forces, motion, and energy one to another in a predetermined manner (2): an instrument (as a lever) designed to transmit or modify the application of power, force, or motion. Which requires definitions that build a bridge between force and power: Force – A push, pull, lift, press, etc. (MKS unit is called a Newton (N): 1 N = 1 kg m/sec2) Work – The increase or decrease in the amount of energy in a system by the action of a force through a distance (MKS unit is called a Joule: 1 J = 1 N m) Energy – The measure of system’s ability to affect change in itself or the environment (MKS unit is also called a Joule (J): 1 J = 1 N m) Power – The rate of work or energy with respect to time (MKS unit is called a Watt (W): 1 W = 1 J/sec) Date Completed: Preliminary Final Grader Name: Initials: Grade: Grade: Initials: EP@BHS-TOPIC 2: Energy, UNIT2.6: Mechanisms Page 3 A simple machine is a device which converts a work input into a useful work output through one of six basic mechanisms, as shown in the illustration to the right. These images follow the convention that the input is referred to as the “effort” and the output is referred to as the “resistance” From the images to the right, it should be evident that a simple machine can multiply force such that Fr is greater than Fe. That is, a small input force (Fe) can accomplish a task requiring a large output force (Fr). But the constraint is that the small input force must be exerted through a larger distance so that the work input is equal to the work output. You are trading a small force acting through a large distance for a large force acting through a small distance. This is the nature of all the simple machines above as they are shown above. The factor by which a machine multiplies the force is called the "mechanical advantage". (Source: http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/simmac.html#c1) For ideal machines, would be no energy loss within the system, the work output will be equal to the work input, and the machine would be 100% efficient: Ideal Machine: No energy is losses occur within the machine Work Input = Ideal Work Output Efficiency = 100% Ideal Mechanical Advantage (IMA) = Ideal Output Force/Input Force For real machines, there will always be some factors (like friction, flexing, etc.) that will convert some of the energy input into non-useful (thermal) energy. This results in the useful mechanical energy output being less than the mechanical energy input and causes the actual mechanical advantage (AMA) to be lower than the ideal mechanical advantage (IMA): Actual Machine: Energy losses occur within the machine Work Input > Actual Work Output Efficiency (always less than 100%) = Actual Energy Output/Energy Input Actual Mechanical Advantage (AMA) = Actual Output Force/Input Force Simple Machine Examples Directions: The following pages include background information on each simple machine and space for students to document an example of each in detail, by utilizing a variety of provided materials from which to construct simple machines and responding to the associated prompts. Students may work together in groups of their own choosing. Students will be provided with a variety of tools to measure associated lengths and forces. Date Completed: Preliminary Final Grader Name: Initials: Grade: Grade: Initials: EP@BHS-TOPIC 2: Energy, UNIT2.6: Mechanisms Page 4 Lever - consists of a lever arm and a fulcrum. Effort (Fe) is the input force which must be supplied Fe Le Lr Fr by the user or an engine of some kind. Resistance (Fr) is the output force which is also the force resisting motion. MECHANICAL ADVANTAGE: 1. IDEAL MECHANICAL ADVANTAGE for a LEVER ideal mechanical advantage (IMA) = Length from fulcrum to effort = Le Length from fulcrum to resistance Lr 2. ACTUAL MECHANICAL ADVANTAGE for a LEVER actual mechanical advantage (AMA) = Resistance = Fr Effort Fe Types of Levers First class lever: the fulcrum is located in the center of the lever arm and the effort and load are at opposite ends. Example Seesaw Second class lever: with a second-class lever the weight is located in the middle and the fulcrum and the effort or at opposite ends. Example Wheelbarrow Third class lever: the effort is applied at the middle of the arm and the weight is held at one end while the fulcrum is at the other end. Example Elbow Bending First Class (see-saw) Second Class (wheel barrow) Third Class (catapult) NOTE: Label important quantities on diagrams: Fe, Le, Fr, Lr Date Completed: Preliminary Final Grader Name: Initials: Grade: Grade: Initials: EP@BHS-TOPIC 2: Energy, UNIT2.6: Mechanisms Page 5 Lever Example Documentation: Perform the following: • Sketch the lever • Label the effort and the resistance • Measure and label quantities necessary to determine the ideal mechanical advantage • Calculate the ideal mechanical advantage • Measure the effort and the resistance associated with using the lever to support a load • Calculate the actual mechanical advantage Date Completed: Preliminary Final Grader Name: Initials: Grade: Grade: Initials: EP@BHS-TOPIC 2: Energy, UNIT2.6: Mechanisms Page 6 Pulley - A single pulley simply changes the direction of a force. When two or more pulleys are connected together, they permit a heavy load to be lifted with a smaller force. The trade-off is that the end of the rope must move a greater distance than the load. Fixed Pulley Moveable Pulley Block & Tackle NOTE: Label important quantities on diagrams: Fe, Fr MECHANICAL ADVANTAGE: 1. IDEAL MECHANICAL ADVANTAGE for a PULLEY ideal mechanical advantage (IMA) = number of strands supporting the resistance 2. ACTUAL MECHANICAL ADVANTAGE for a PULLEY actual mechanical advantage (AMA) = Resistance = Fr Effort Fe Fixed Pulley: The pulley is attached or fixed to a strong member, which will not move. When a fixed pulley is used the force needed to lift a weight does not change and the rope must be pulled the same distance as the weight is lifted. Movable Pulley: The effort needed to lift 180 pounds weight is 90 pounds. The mechanical advantage of a movable pulley is 2 and the rope must be pulled twice the distance the weight is lifted Block and
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