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SIFT: Mechanical Comprehension: Pick the Right Principle

SIFT · Work through your exam, one topic at a time.

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Mechanical Comprehension: Pick the Right Principle

What is being tested

Everyday physics: levers, pulleys, gears, inclined planes, work and power, pressure, and simple fluid systems. The arithmetic is light; the challenge is recognizing which principle applies.

Core concepts

  • Levers: mechanical advantage equals effort arm divided by load arm, both measured from the fulcrum. A longer effort arm means less force but more distance moved. Classes: first (fulcrum between load and effort), second (load between), third (effort between).
  • Pulleys: a single fixed pulley changes direction only, with a mechanical advantage of 1. Movable pulleys and systems supported by n rope segments give advantage n, but you pull n times as much rope.
  • Gears: meshing gears turn in opposite directions; the larger gear turns slower with more torque. Ratio equals teeth on the driven gear divided by teeth on the driver.
  • Work, power, energy: work equals force times distance in the direction of the force; power equals work divided by time. Machines trade force for distance but do not reduce total work, ignoring friction.
  • Pressure: pressure equals force divided by area, so a small area produces high pressure, which is why a sharp edge cuts.
  • Fluids: in a confined liquid, pressure is transmitted equally in all directions; hydraulic advantage comes from the ratio of piston areas.

Traps

  • Confusing force with work: holding a heavy box motionless does no work.
  • Assuming a pulley system reduces the work required; it only reduces the force.
  • Forgetting that friction always reduces real mechanical advantage.
  • Mixing up which gear is the driver in a ratio question.

Worked example

A lever has its fulcrum 2 feet from a 300 pound load, and effort is applied 6 feet from the fulcrum. Mechanical advantage is 6 divided by 2, or 3, so the required effort is 300 divided by 3, or 100 pounds. Check: the effort end moves three times as far as the load.

Rule of thumb

Ask what is being traded. In every simple machine, a smaller force is paid for with a longer distance.