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List ALL Assumptions

1- Read Carefully

2- Draw a Diagram

3- List Given Information

4- List All Assumptions

Write Equations & Lookup Data

6- Solve Equations

Verify Assumptions

8- Answers

This stairway shows that the solution for a problem is a logical sequence of steps.

4- List All Assumptions

Making valid assumptions is the key to solving most engineering problems. A good assumption is one that simplifies the problem without significantly reducing the accuracy of the solution.

It is very important that you make a list of all of the most significant assumptions upon which your solution is based. Assumptions regarding things like the curvature of
the Earth and the acceleration of gravity can frequently be omitted from
this list unless they play a key role in the solution of the problem.

The justifcation for each assumption should be given as well. In many cases, the justification for an assumption will
consist of a short explanation. However, some
assumptions cannot be justified until the
problem has been solved. See
Step 7 for more details.

Lesson
4D Blog

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Lesson 4D: page 6 of 13

Making the right set of valid assumptions is something of an art.
For example, it is often, but not always, valid to assume that the gravitational acceleration at the surface of the Earth is 9.8066 m/s2, even though you are aware of the fact that the gravitational acceleration is not the same at all locations on the Earth's surface.
The variations in g are often too small to effect the 2 or 3 significant figures that are appropriate for most engineering problems.
In this step, you must list all of the most important assumptions that you are making about the system and process that you are analyzing.
For most problems, an object not moving relative to the Earth is considered stationary, despite the fact that the Earth is rotating about its axis and revolving around the sun.
Assumptions like this do not need to be listed in this step.
Assuming that a process is isothermal or adiabatic is a very significant assumption and it should be listed in this step.
Each assumption listed here should be justified.
Most justifications consist of one short sentence.
Some assumptions cannot be justified until the problem has been solved.
Suppose you assume a gas is ideal without knowing its molar volume.
This type of assumption will be discussed in more detail when we reach step 7.

List ALL Assumptions

Ch 4, Lesson D, Page 6 - List ALL Assumptions