AMIE Section B Mechanical common mistakes
AMIE Section B Mechanical · Understand the traps and how to avoid them.
Using the wrong sign convention or drawing the shear force and bending moment diagrams without consistent sign rules, so the BM diagram does not close to zero at the free end and the maximum moment location is misidentified.
Candidates memorize the shapes of SFD/BMD for standard cases but never fix a single sign convention (sagging positive, upward loads positive), so when a point load, UDL and moment couple are combined the diagram is assembled from memory rather than from equilibrium checks.
What to do instead
Before starting any beam problem, write the convention on the answer sheet. Compute reactions by taking moments about one support, then verify with vertical force balance. Draw the SFD first and check that its area equals the change in BM between points; confirm the BMD closes at the far end and that BM is zero at a free end and at a hinge. Mark the point of zero shear as the maximum BM location.
Skipping the free body diagram or control-volume sketch and substituting numbers directly into a formula, which leads to using the wrong area, wrong lever arm, or wrong pressure reference.
Under time pressure candidates treat the sketch as decoration and jump to the equation, and because many AMIE problems are deliberately similar to textbook cases they assume the standard geometry instead of reading the actual dimensions and support conditions given.
What to do instead
Make a labelled free body diagram mandatory for every equilibrium, stress, and fluid problem: show all forces, distances, areas, and the assumed positive direction. For fluids, draw the control volume and mark the datum line for pressure and Bernoulli terms. Only after the diagram is labelled should any equation be written.
Failing to convert units consistently, mixing mm and m, kW and W, bar and N/mm2, or rpm and rad/s within a single calculation, and reporting answers without units.
The paper mixes SI and practical units (N/mm2 for stress, bar for pressure, kW for power, rpm for speed) and candidates carry numbers across formulas without tracking the unit of each symbol, especially in power and torque relations where the 2*pi/60 factor is easy to drop.
What to do instead
Write the unit beside every substituted value. Convert everything to a coherent set (N, m, Pa, rad/s, W) at the start, then convert the final answer to the requested unit. Memorize key bridges: 1 bar = 100 kPa = 0.1 N/mm2, 1 N/mm2 = 1 MPa, P = 2piN*T/60 with T in N.m and N in rpm. Always append units to the final answer.
In thermodynamic cycle and heat transfer problems, applying the first law without checking whether the process is specified as reversible, steady-flow, or closed-system, and using the wrong specific heat (cp versus cv) or the wrong area and resistance network.
Candidates pattern-match on the word 'efficiency' or 'heat' and reach for a memorized formula, without classifying the system (open versus closed), the process (isothermal, adiabatic, polytropic), and whether the substance is a gas or a two-phase mixture.
What to do instead
For every problem first state: system boundary, steady or unsteady, substance and phase, and the process path. Then select the energy equation accordingly (Q - W = dU for closed; steady-flow energy equation for open). Use cp only for constant-pressure processes and cv for constant-volume. For heat transfer, draw the thermal resistance network, identify series versus parallel paths, and check whether the surface is finned or the Biot number justifies lumped analysis.
Making arithmetic and algebra errors in the design of machine elements by omitting the factor of safety, using the wrong stress concentration factor, or forgetting to check the combined stress (principal stress or equivalent stress) instead of a single direct stress.
Design problems involve long chains of substitution (torque, bending moment, equivalent moment, allowable stress), and candidates stop at the first stress value without verifying that the diameter satisfies fatigue, bearing, and keyway checks; they also forget that allowable stress = yield or ultimate divided by factor of safety.
What to do instead
Structure the answer as a checklist: determine loads, compute direct and bending stresses separately, combine using the appropriate theory (maximum shear or von Mises), apply the stress concentration factor to the alternating component, apply the factor of safety to get allowable stress, then solve for the dimension. Do a sanity check on the final diameter against typical ranges for the application.
Treating manufacturing process questions as pure recall and giving a generic process name instead of matching the process to the material, batch size, tolerance, and surface finish required, or confusing terms such as forging versus extrusion, or brazing versus soldering.
Candidates memorize definitions in isolation and do not build a decision framework linking material properties, production volume, and required accuracy, so they cannot discriminate between two plausible processes in a comparative question.
What to do instead
Build a comparison table for each process family: material compatibility, achievable tolerance, surface finish, tooling cost, and suitable batch size. In answers, justify the choice with at least two of these criteria. Practice discriminating pairs (hot versus cold working, brazing versus soldering by temperature, casting versus forging by shape complexity).
In engineering drawing and graphics, drawing the orthographic views without establishing the correct alignment and projection convention, omitting hidden lines, or misreading the given view so the third-angle or first-angle layout is wrong.
Candidates learn the symbols for first-angle and third-angle projection but do not practice the discipline of aligning views on projection lines and transferring dimensions consistently, so the front, top, and side views are drawn as independent sketches rather than as one consistent object.
What to do instead
Before drawing, decide the projection system and state it. Lay out the three views on a grid so that the front and top views share width, and the front and side views share height. Transfer depths with a 45-degree mitre line. After completing, verify that every feature visible in one view appears as a solid or hidden line in the others, and that no line contradicts another view.
In fluid mechanics and hydraulic machinery, ignoring losses and efficiency by applying Bernoulli's equation across a pump or turbine, or using the wrong velocity triangle and blade angle relations for pumps and turbines.
Candidates apply the ideal Bernoulli equation everywhere because it is the most familiar formula, forgetting that it holds only along a streamline of an ideal fluid without work transfer, and they confuse the head, discharge, and specific speed relations for pelton wheel, Francis, and Kaplan machines.
What to do instead
State the assumptions of Bernoulli's equation each time (steady, incompressible, frictionless, no work). For machines, use the energy equation with head added or extracted and include efficiency. Draw the velocity triangles at inlet and outlet, label absolute, relative, and blade velocities, and use the Euler head equation. Check that specific speed ranges match the type of machine selected.
In industrial engineering and management, computing EOQ, break-even, or network and scheduling problems with the wrong cost basis, forgetting holding cost per unit per year versus per order, or misapplying the critical path versus the longest path in a project network.
Candidates memorize the EOQ formula but not the meaning of each symbol, and in PERT/CPM they confuse float with slack or assume the longest path is automatically critical without checking for dummy activities and multiple paths.
What to do instead
Write the definition of each cost term before substituting (ordering cost per order, holding cost per unit per year, demand per year). For networks, list all paths explicitly, compute each path length, identify the critical path as the longest, and compute total float as latest start minus earliest start for every activity. Verify that the sum of floats on the critical path is zero.
In electrical fundamentals and computer programming, confusing the phasor and instantaneous domains, mixing rms and peak values, or writing a program that runs but does not handle the boundary cases and data types the question asks about.
Candidates treat AC circuits as DC circuits with resistors only, ignoring phase and impedance, and in programming they write code to satisfy the sample input without testing edge cases such as zero, negative, or empty input, or they confuse integer and floating-point division.
What to do instead
For AC problems, convert all sources to phasors, compute impedance as R + jX, and keep track of rms versus peak throughout; state the reference phase. For programming, write pseudocode first, identify the data types and boundary conditions, then trace the code by hand with one normal and one edge case before writing the final answer.