1. Class 7_1 Today’s topic: More on engineering analysis and modeling Building a little example model

2. DP questions?

3. Monitors off, please

4. All engineering analyses are Models

5. What is a model? A purposeful representation of some part of the real world. Real system Model Computer Modeling Simulation

6. Tips for making engineering models Make simplifying assumptions. What physical laws are relevant? Draw a diagram. Break the problem in to parts. Look for upper and lower bounds.

7. Let’s try it for a previous project The problem…a rubber band powered car pushing a block (see blackboard)

8. Can we improve the model? How?

9. Tips for making engineering models Make simplifying assumptions. What physical laws are relevant? Draw a diagram. Break the problem in to parts. Look for upper and lower bounds.

10. Things to notice from this example No high-powered physics or math was needed, but did require careful application of basic principles and judicious assumptions We used MatLab to exercise the model, and extract useful information The model revealed behavior (i.e., a wheel/spool ratio that was optimum) we could not have guessed or likely found by trial/error.

11. Today’s model is on the web To see the analysis worked out carefully, please look at the class website.

12. This model is not sufficient for your analysis! It gives you a place to start, but your situation is different. You must build your own model for the particular circumstances of your design.

13. Please start doing your analysis Start now Study examples on class web page Next we will review MatLab programming (You do not need this to start your analysis) Come see me if you need help

14. This is a chance to take what you have learned and do some real engineering! Good luck and have fun with it!

15. RUBBER BAND DATA deflection=[0,.01,.02,.05,.07,.1,.12,.15,.17,.2]; %units m force=[0,5.7,8.5,14.3,20.0,27.3,32.1,39.1,48.4,71.6]; %units N After being subjected to several cycles, the rubber band becomes much more likely to break when stretched beyond 15 cm. Initial values for the various parameters. d_2=6; %drive wheel diameter (cm) d_1=1; %spool diameter (cm) m_c=.750; %mass of car (kg) m_b=.600 ; %mass of block (kg) c_friction=.9; %coeff of car friction b_friction=.3; %coeff of block friction k=249.2; %rubber band spring constant (N/m) g=9.81; %gravitational constant F_break=39.1; %breaking force of the rubber band (N)