Mechanical Comprehension
What to know
Mechanical Comprehension asks how forces, motion, and simple machines behave. Master mechanical advantage, levers, pulleys, gears, inclined planes, torque, pressure, and friction.
Levers. First class: fulcrum between effort and load (seesaw). Second class: load between fulcrum and effort (wheelbarrow). Third class: effort between fulcrum and load (tweezers). Mechanical advantage equals effort arm divided by load arm.
Pulleys. A fixed pulley only changes direction. A movable pulley halves the effort, and advantage equals the number of rope segments supporting the load.
Gears. Meshing gears turn in opposite directions. The larger gear turns more slowly and delivers more torque. Gear ratio equals driven teeth divided by driving teeth.
Pressure equals force divided by area, so a narrow tip concentrates force. Torque equals force times distance from the pivot.
Traps
- Counting pulleys instead of the rope strands supporting the load.
- Forgetting that meshing gears reverse direction.
- Assuming a longer lever needs more force; it needs less.
- Mixing torque with pressure, or force with work.
Rule of thumb
For any machine, compare the distance the effort moves with the distance the load moves. Whatever you gain in force, you pay back in distance.
Worked example
A lever has a 2 m effort arm and a 0.5 m load arm. Mechanical advantage = 2 ÷ 0.5 = 4. A 50 N push lifts a 200 N load, but the effort end must travel four times as far as the load rises.