How DAA Mechanical Comprehension works
The Royal Navy lists two areas: moving systems and machinery, such as cogs, gears, pulleys and springs, and static mechanics, such as balance, force and mass.
The official familiarisation mixes reasoning about machines, such as which way each gear in a train turns, with plain workshop knowledge, such as what a scale on an engineering drawing means or which tool tightens nuts and bolts, and a question on gas pressure in a cylinder.
How to answer mechanical questions
Name the mechanism first. Each one has a rule that settles most questions:
- Gears: meshed gears turn in opposite directions, and a smaller gear turns faster than the larger gear driving it.
- Levers: load times its distance from the pivot equals effort times its distance.
- Pulleys: the more rope sections holding the load, the less effort you need and the more rope you pull.
- Gas in a sealed cylinder: squeeze it into less space and the pressure rises.
- When answers come as a set of directions, rule out any set that is wrong on the first gear before checking the rest.
Worked example
A beam rests on a pivot. A 600 N load sits 0.5 m to the left of the pivot. “What force, 1.5 m to the right of the pivot, balances the beam?” Options: 100 N, 200 N, 300 N, 900 N, 1,800 N.
The turning effect on the left is 600 × 0.5 = 300 newton-metres. To match it, the force × 1.5 must also be 300, so the force is 300 ÷ 1.5 = 200 N. 900 N is the trap: it multiplies by the distance instead of dividing.
Tips for the real Mechanical Comprehension section
- Do not skip workshop topics. Tools and drawing scales appear in the official examples and are quick marks if you know them.
- Write gear directions on paper as you go, rather than holding a whole chain in your head.
- With about 41 seconds a question, leave any long drawing for the end and come back to it from the summary.
Practise DAA Mechanical Comprehension on CogniPrep
Our Mechanical Comprehension practice is 19 questions in 13 minutes, with five options, a visible clock, Back and Next, and a summary before you submit. The questions and drawings are ours, written to the topics the Royal Navy lists, and each sitting draws 19 from a larger bank with no topic over a quarter of the paper. Calculations use whole numbers, nothing is deducted for wrong answers, and you get your mark out of 19.