Mechanical Reasoning
Core concepts
Mechanical reasoning questions ask how simple machines and physical systems behave. Know the three lever classes, pulley systems, gear trains, mechanical advantage, friction, pressure (force ÷ area), and the trade-off between force and distance. In an ideal machine, work input equals work output: effort × effort distance = load × load distance. Mechanical advantage (MA) is load ÷ effort, or effort distance ÷ load distance for a lever. For gears, meshing gears turn in opposite directions; a larger driven gear turns slower and with more torque.
Traps
Do not assume more pulleys always means less effort without checking which pulleys are movable. Do not confuse speed with force: a smaller driving gear makes the driven gear turn slower, not faster. In lever problems, measure distances from the fulcrum, not from the end of the bar. Pressure problems often mix total force with force per unit area; a larger contact area lowers pressure for the same force. Friction is usually ignored unless stated, but real systems lose efficiency.
Worked example
A 200-pound load sits 2 feet from the fulcrum of a lever. How much effort is needed 8 feet from the fulcrum on the other side? Effort × 8 = 200 × 2 Effort = 400 ÷ 8 = 50 pounds. The lever gives a mechanical advantage of 4. Trap: using 8 − 2 = 6 feet as the effort arm; distances are measured from the fulcrum.
Rule of thumb
Draw the pivot, label every distance from it, and balance moments: effort × effort arm = load × load arm. Then ask whether the machine trades force for distance.