The Systems Zoo EIND 142 Lecture 2 1 Overview • Examine examples of systems to illustrate key points • Two competing loops 2 Two competing balancing loops 3 Start with cold room at 10 C and set thermostat to 18 C However heat leaks from the room- it is in the larger system 4 Feedback loop concepts • The information delivered by a feedback loop can only affect future behavior; it can’t deliver the information, and so can’t have an impact fast enough to correct behavior that drove the current feedback. • Why is that important? Because it means there will always be delays in responding. It says that a flow can’t react instantly • Many economic models make a mistake in this matter by assuming that consumption or production can respond immediately to a change in price for example. 5 What is the difference - what is the problem? 6 Business and economic examples • Now we add reinforcing and balancing loops This reflects birth rate greater than mortality rate. What could happen to change these predictions? 7 Conclusions about loops A stock governed by linked reinforcing and balancing loops will: • grow exponentially if the reinforcing loop dominates the balancing one. • It will die off if the balancing loop dominates the reinforcing one. • It will level off if the two loops are of equal strength (see Figure 25). • Or it will do a sequence of these things, one after another, if the relative strengths of the two loops change over time (see Figure 26). 8 Three questions to ask • Are the driving factors likely to unfold this way? (What are birth rate and death rate likely to do?) • Crystal ball: Testing scenarios example • If they did, would the system react this way? (Do birth and death rates really cause the population stock to behave as we think it will?) • Is the model credible? How to evaluate? • What is driving the driving factors? (What affects birth rate? What affects death rate?) • Many factors influence these rates. 9 It’s the economy, stupid! • Capital stock is machines, raw materials, infrastructure, intellectual capital, etc. 10 Influences • Our investment rate and our competitive position in the life of our capital assets: The world is flat! 11 Business Inventory- delays 12 System with delays in three places 13 Bullwhip effect • A single step up in sales causes inventory to drop. The car dealer watches long enough to be sure the higher sales rate is going to last. • Then begins to order more cars to both cover the new rate of sales and bring the inventory up. But it takes time for the orders to come in. • During that time inventory drops further, so orders have to go up a little more, to bring inventory back up to ten days’ coverage. • Eventually, the larger volume of orders starts arriving, and inventory recovers—and more than recovers, because during the time of uncertainty about the actual trend, the owner has ordered too much. • Dealer now sees the mistake, and cuts back, but there are still high past orders coming in, so dealer orders even less. • In fact, almost inevitably, since still can’t be sure of what is going to happen next, dealer orders too little. • Inventory gets too low again. And so forth, through a series of oscillations around the new desired inventory level. 14 Homework • Consider the diagram on the economy Figure 27. 1. Identify two areas ( and briefly explain) where you as an engineer and citizen can grow US capital assets (stocks). 2. What are two areas we are not doing well ( and briefly explain)? 3. What is one key area where a delay arrow should be shown- explain this delay and its impact. • Submit your thoughts in a professional manner. 15 EIND 142 Homework Standards All homework problems, unless otherwise directed by your instructor, should follow these guidelines. In general, homework may be computational or written. Computational format: This format applies for most professional engineering work. Unless otherwise directed by your instructor, you should use engineering paper or the equivalent for all computational homework assignments. Write only on the front side of the paper (the dark green grid should be on the back). Headers: The boxes at the top of each sheet of engineering paper that you use for a homework assignment should contain the following information from left to right: 1. Print your full name in the first (large) box. If this is a team homework, then print the team leaders name in the fourth box and the names of each participating team member below this box. 2. Print the course and section number in the second (large) box 3. Print the assignment in the third (large) box. 4. Print the page number / total number of pages in the fifth (small) box (1/5, 2/5, etc. Writing Mechanics: All homework should be: 1. Carefully printed and not written in cursive 2. Printed in pencil and not in ink, 3. Neat and clean, i.e. printed on the lines with no smudges or cross-outs Calculations: All homework calculations should: 1. Include the core equation in general terms and then follow with the numerical version 2. Include all units for each term in each equation and the units must balance 3. Use the appropriate number of significant figures (usually three) for all numbers 4. Clearly indicate the final solution by boxing it in with a rectangle Problem Essentials: Problem solutions should include the following items in order: 1. Homework problem number listed at beginning of problem and the problem statement 2. The given information - the information that will be used to solve the problem 3. The required information - the information or solution that we are looking for 4. The solution of the problem including all required steps and calculations Computers: Homework Assignments using Computers 1. Show sample calculations (with units) for each spreadsheet calculation Make sure results are fully explained and integrated into the homework document. 2. Be sure you submit sufficient information so you demonstrate computer and interpretation skills and fully answer the question. Essay or written Homework Generally, essay or written homework follows the requirements above with additions: Headers: make sure each page is identified and numbered as noted above. Format: Should be word-processed without grammar or spelling issues and figures / tables should be as professional as possible. Use times new roman, 11 pt, 1.5 line spacing and1 inch margins. Identify each page as noted for the computational homework. Problem identification: Clearly describe the question and follow this with your solution or answer Figures or Tables: Unless specifically noted in the instructions, figures and tables should not be manual but word processed, copy / paste, or computer generated. They should be numbered, titled, and fully explained in the problem answer.
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