Mechanical FE Exam: Common Mistakes
5 costly errors to avoid on the FE Mechanical Exam
Mistake #1: Mixing up sign conventions for work and heat in thermodynamics
Adopt one sign convention and stick with it throughout the problem. The most common convention: heat added to the system is positive, work done by the system is positive. Write your convention at the top of your scratch work. Many wrong answers come from flipping signs mid-calculation.
Mistake #2: Forgetting to account for all forces when drawing free body diagrams
Systematically check for gravity, applied loads, normal forces, friction, spring forces, and reaction forces at every support. Missing a single force changes the equilibrium equations entirely. Take 30 extra seconds on the FBD to save minutes of rework later.
Mistake #3: Confusing absolute and gauge pressure in fluid mechanics problems
Always identify whether the problem gives absolute or gauge pressure. Absolute pressure equals gauge pressure plus atmospheric pressure. Using gauge pressure where absolute is required (especially in ideal gas law calculations) produces significantly wrong results.
Mistake #4: Using the wrong moment of inertia in bending stress calculations
Bending stress calculations require the moment of inertia about the neutral axis, not the centroidal axis of individual parts. For composite cross-sections, use the parallel axis theorem to transfer each part's moment of inertia to the neutral axis before summing them.
Mistake #5: Neglecting to check Reynolds number before applying fluid flow correlations
Laminar and turbulent flows use different friction factor and heat transfer correlations. Always calculate the Reynolds number first to determine the flow regime. Applying a turbulent correlation to laminar flow (or vice versa) gives answers that may differ by an order of magnitude.
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