Mechanical Comprehension
What to master
Know how simple machines trade force for distance: levers, pulleys, inclined planes, wedges, screws, and wheel-and-axle systems. Understand mechanical advantage, torque, work and energy, pressure, buoyancy, friction, and gear direction and speed. Many questions ask which setup requires less effort or moves faster.
Traps to expect
- Force versus distance: a machine can reduce force but then requires more distance; work input is not reduced.
- Lever arms: mechanical advantage equals effort arm divided by load arm, measured from the fulcrum-not the total board length.
- Pulley count: count the rope segments supporting the movable load, not every pulley in the drawing.
- Gear direction: meshed gears rotate in opposite directions; a belt without a twist makes pulleys turn the same way.
- Mass versus weight: mass is constant; weight depends on gravity.
Worked example
A lever has a 6-foot effort arm and a 2-foot load arm. Mechanical advantage = 6 ÷ 2 = 3. A 300-pound load therefore needs about 100 pounds of effort, ignoring friction. The trade-off is that the effort end moves three times as far as the load.
Rule of thumb
Use conservation of energy: force × distance is constant in an ideal machine. If a device multiplies force by 3, it moves the load one-third as far. Check units and identify the pivot or supporting rope segments before choosing an answer.