Mechanical Comprehension
Mechanical Comprehension tests everyday physics: levers, moments, pulleys, gears, pressure, friction, and basic force balance. Questions often describe a tool or simple machine and ask for the effort, direction, or mechanical advantage.
Core rules
- Moment = force x perpendicular distance from the pivot. Balance occurs when clockwise moments equal anticlockwise moments.
- Lever classes: first class has the pivot between load and effort; second class has the load between pivot and effort; third class has the effort between pivot and load.
- Mechanical advantage = load divided by effort. It trades distance for force, not free energy.
- A single fixed pulley changes direction but not ideal effort. A movable pulley can halve the effort, at the cost of pulling twice the rope length.
- Meshing gears turn in opposite directions; larger gears turn more slowly than smaller gears.
- Pressure = force divided by area. The same force over a smaller area gives higher pressure.
Worked example
A load weighing 600 N sits 1.5 m from a lever's pivot. An effort is applied 3.0 m from the pivot. Ignoring the beam's weight, what effort balances the load? Clockwise moment = 600 x 1.5 = 900 N m. For balance, effort x 3.0 = 900, so effort = 300 N. The mechanical advantage is 600 divided by 300 = 2.
Traps
- Measuring distance from the beam end instead of the pivot.
- Treating mass in kilograms as force without converting to newtons.
- Forgetting that gears reverse direction at each mesh.
- Assuming a pulley system removes energy rather than redistributing force and distance.