Mechanical Comprehension
What is tested
Mechanical comprehension asks you to predict how simple machines and physical systems behave: levers and moments, pulleys, gears, inclined planes, pressure, buoyancy, friction, springs, and centre of gravity.
Core rules
- Moments: a lever balances when force × distance from the pivot is equal on both sides. A small force far from the pivot can lift a large load close to it.
- Gears: meshing gears turn in opposite directions. A 12-tooth gear driving a 36-tooth gear turns three times for every single turn of the larger one; the larger gear turns more slowly but with greater torque.
- Pulleys: a single fixed pulley changes direction only, giving no mechanical advantage. A movable pulley halves the effort; count the rope sections supporting the load.
- Pressure = force ÷ area, so a sharp edge concentrates force.
- An object floats if it displaces its own weight in fluid.
Traps
- Confusing the effort distance with the load distance.
- Assuming a bigger gear turns faster (it turns slower but with more torque).
- Treating mass and weight as identical, or ignoring the direction of forces.
- Forgetting friction, or assuming a pulley system gives free advantage.
Worked example
A see-saw has a 60 kg child sitting 2 m from the pivot. Where must a 40 kg child sit to balance it? Moments: 60 × 2 = 120 kg·m. So 40 × d = 120, giving d = 3 m. The lighter child sits further from the pivot.
Rule of thumb: for any balance question, write 'force × distance' for each side, set the two sides equal, and solve for the missing value.