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Problem Solving Engineers of all disciplines are often challenged with unfamiliar problems By breaking them down into a systematic methodology, many “impossible” problems can be solved By systematizing your approach, you will leave an “auditing” trail for all those who later work on the same project. As a huge bonus, the suggested method really helps in getting a good grade!
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Exploring EngineeringChapter 3, Part 1
Engineering Problem Solving
What You Will Learn
A formal technique to help you solve problems
- “Need-Know-How-Solve” method breaks the problem done into four constituent parts that are easier to formulate than overall problem.
Problem Solving Engineers of all disciplines are often
challenged with unfamiliar problems By breaking them down into a systematic
methodology, many “impossible” problems can be solved
By systematizing your approach, you will leave an “auditing” trail for all those who later work on the same project.
As a huge bonus, the suggested method really helps in getting a good grade!
The “Need-Know-How-Solve” Method
Need: The 1st step is obvious: read the problem
very carefully. Look for what is being sought. Don’t try to solve it now. Just write down what you are seeking.
Know: Look at what you have been given (or look it
up in available resources if not explicit in the statement of the problem). Again, don’t try to solve it now. Just write down what you know as the 2nd step.
Solve:The 4th and last step does what your
instincts told you (incorrectly) to try as step 1: go ahead and get to a solution.
How:The 3rd step formulates your intended approach. It may be trivial (e.g., how many apples for $1?) or it may be an equation (e.g., E = mc2) or it the need for a spreadsheet analysis etc. Still don’t try to solve it now.
Stress is defined as the force/area. Calculate the stress in SI units in an 0.50 inch diameter cable supporting a 1,000. lbm truck engine. Need: Stress, symbol Greek sigma , in cable
– Know: Force, F = 1,000. lbf = 4450. N (Convert.exe) and diameter is 0.50 inch– How: Stress F/A, where A = R2 = D2/4– Solve: A = 3.14 (0.50 0.0254)2/4 [in]2[m/in]2 = 5.07 10-4 m2
Hence = F/A = 4450./5.07 10-4 [N]/[m2] = 8.8 106 N/m2
– Hopefully less than the breaking stress in the cable.
0.5 tons0.5 ton engine
Cab
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0.5 tons0.5 ton engine
Cab
le
Example
The “Need-Know-How-Solve” Method
On a single lane highway, you measure that there are 3140 cars/hr passing under a bridge. What is the separation between cars in seconds? Need: Spacing in time between cars Know: 3140 cars/hr How: Dimensional analysis based on […] units Solve: If 3140 cars/hr, time in s = 3600/3140
[s/hr][hr/car] = 1.15 s/car (to 3 significant figures)
If, in the previous example, the cars are traveling at 69 mph, what is their separation in m? In approximate car lengths? Does this meet a 1 car length per 10 mph spacing? Need: Spacing between cars Know: 3140 cars/hr, interval = 1.15 s and v = 69 mph How: […] method. v = 69 mph = 30.8 m/s. Assume
average car is ~4. m long. Solve: Since t = 1.15 s/car and v = 30.8 m/s,
distance/car = 30.8 1.15 [m/s][s/car]= 35.3 m of which 4. m. is car length.
Spacing = 31 m or 31/4 [m/car][car/m] = 7.8 ~ 8 (car lengths), which is greater than the recommended 7 car lengths.
More
The “Need-Know-How-Solve” Method
Summary:1) Engage the mind before the pencil!2) Delay solution until you have all in the facts.3) Allow for a traceable solution for other
members of an team (warts and all!).4) As a practical matter, you can get most of
the grade for the same wrong answer if you follow this methodology!
– E.g, write just the answer as “84.7” and may get you “0” grade but not for a clear development to a solution that said T = 8670/10.0 = 84.7! You would still get most of the grade. (This mimics the auditing trail required of a practicing engineer.)
