Mechanical Comprehension: Forces, Machines, and Motion
Mechanical Comprehension tests intuitive physics: how forces, levers, gears, pulleys, and fluids behave in real devices. You rarely need heavy math-you need to reason about how a system responds.
Core concepts
- Force and motion: a net force causes acceleration; friction opposes motion; heavier objects need more force to move.
- Levers: a lever pivots on a fulcrum. A longer effort arm gives more mechanical advantage. Classes differ by where the fulcrum, load, and effort sit.
- Pulleys: a fixed pulley changes direction only; a movable pulley reduces effort by sharing the load among supporting rope segments.
- Gears: meshed gears turn in opposite directions; a larger gear turns slower but with more torque; the gear ratio sets the speed/torque trade-off.
- Inclined planes and wedges trade distance for force.
- Fluids: pressure increases with depth; liquids are nearly incompressible; hydraulic systems multiply force.
Traps
- Assuming more speed means more force-often the opposite (gear ratios).
- Forgetting direction-gears reverse direction each mesh.
- Confusing effort and load arms on levers.
- Ignoring friction or weight in idealized diagrams.
Worked example
A lever has a 2-foot effort arm and a 1-foot load arm. What effort lifts a 100-lb load? Mechanical advantage = effort arm ÷ load arm = 2 ÷ 1 = 2. Effort = load ÷ MA = 100 ÷ 2 = 50 lb. The longer effort arm halves the force needed-but you must push through twice the distance.
Rule of thumb
Machines trade force for distance. If a device multiplies force, it requires more movement-energy is conserved.