Mechanical Comprehension
What this section tests
Mechanical comprehension items ask you to reason about simple machines and physical relationships: levers, pulleys, gears, inclined planes, wheel-and-axle, plus work, power, pressure, and basic force balance. Most items are qualitative or need only simple arithmetic. The underlying idea is that a machine trades force for distance - it multiplies effort but never reduces the work required.
Core rules
- Lever balance: effort × effort arm = load × load arm, with distances measured from the fulcrum. A longer effort arm means less force needed.
- Pulleys: a single fixed pulley changes direction only. A movable pulley halves the effort but you pull twice the rope. Count the rope segments supporting the movable block to get mechanical advantage.
- Gears: the ratio of teeth determines the ratio of speeds, and meshed gears turn in opposite directions.
- Inclined plane: a longer, gentler ramp needs less force over a longer distance.
- Work = force × distance; power = work ÷ time. Pressure = force ÷ area, so a smaller area gives greater pressure for the same force.
Worked example
A 200-pound load sits 3 feet from the fulcrum of a bar. How much effort is needed 6 feet from the fulcrum on the other side? Balance the moments: 200 × 3 = F × 6, so 600 = 6F and F = 100 pounds. The effort is halved because the effort arm is twice the load arm, but the effort end must travel twice as far. Work in equals work out; nothing is free.
Traps that cost marks
- Measuring lever arms from the end of the bar instead of from the fulcrum.
- Assuming a pulley reduces effort when it is only a fixed pulley redirecting the pull.
- Forgetting that meshed gears reverse direction.
- Believing a machine reduces total work; it only redistributes force and distance.
- Mixing pounds of force with pounds of mass, or ignoring unit mismatches between feet and inches.