Mechanical Reasoning: Force, Motion, and Simple Machines
Concepts to master
Mechanical reasoning questions test intuitive physics rather than formulas. Know levers (first, second, and third class), pulleys (fixed, movable, and block-and-tackle), gear trains, inclined planes, wheel and axle, and pressure and fluids. Understand the tradeoff at the heart of every simple machine: what you gain in force you pay for in distance, and vice versa.
Traps to watch for
- Gear direction and speed. Two meshed gears turn in opposite directions; an idler gear reverses direction without changing the overall ratio. A larger driven gear turns slower and delivers more torque - not faster.
- Mechanical advantage confusion. More pulleys supporting the load reduce the effort needed, but the rope must be pulled farther.
- Fixed pulleys. A single fixed pulley changes the direction of effort only; it gives no force advantage.
- Lever arm measured wrong. Distance is always measured from the fulcrum to the point where the force acts, perpendicular to the force.
- Pressure intuition. Force spread over a larger area produces less pressure; that is why a nozzle tapers.
Rule of thumb
For levers use effort × effort distance = load × load distance. To diagnose any machine, ask: which part moves farther? That part carries the smaller force.
Worked example
A 100-pound load sits 2 feet from the fulcrum of a pry bar. How much effort is needed 8 feet from the fulcrum on the other side? Effort × 8 = 100 × 2, so effort = 200 ÷ 8 = 25 pounds. The effort moves four times as far as the load, which is exactly why it can be one-quarter the size.