How to solve: Mechanical Reasoning
What this category tests
Tests intuitive physics applied to simple machines — levers, gears, pulleys, and fluid pressure — by asking you to trace cause and effect through a mechanical system.
Where this typology comes from
The standard instrument here is the Bennett Mechanical Comprehension Test (BMCT), developed by psychologist George K. Bennett and published in 1940 by The Psychological Corporation. It became a fixture of industrial and military selection for decades, revised in 1969 and again as the BMCT-II in 2014, and its lever/gear/pulley diagram format is still the template most mechanical-reasoning items follow today.
How to approach it
Identify the single physical principle at play in each part of the system — lever balance, gear-ratio inversion, Pascal's principle for fluids, series versus parallel circuits — rather than trying to reason about the whole mechanism at once. Trace the force, motion, or current step by step through the system in the order it actually propagates. Remember that adjacent, meshed gears always rotate in opposite directions, and that mechanical advantage always trades off against distance or speed, never both.
Worked example
Two gears are meshed together. The smaller gear has 10 teeth and rotates at 60 revolutions per minute (RPM). The larger gear has 30 teeth. How fast does the larger gear rotate, and in which direction relative to the smaller gear?
- 20 RPM, same direction
- 20 RPM, opposite direction ✓ Correct answer
- 180 RPM, opposite direction
- 60 RPM, opposite direction
Explanation Meshed gears always rotate in opposite directions. For speed, the product of teeth × RPM stays constant across meshed gears: 10 teeth × 60 RPM = 600 = 30 teeth × RPM, so the larger gear turns at 600 ÷ 30 = 20 RPM. A gear three times larger always turns three times slower.
Practice Mechanical Reasoning
- Mechanical Reasoning I — Levers, Gears, and Pulleys · 12 questions · Difficulty 2 out of 5
- Mechanical Reasoning II · 12 questions · Difficulty 3 out of 5
- Mechanical Reasoning III — Fluids and Structural Stability · 12 questions · Difficulty 4 out of 5