DAA Mechanical Comprehension: gears, levers and pulleys explained

The Mechanical Comprehension section of the Defence Aptitude Assessment (DAA) checks how well you understand everyday physics and the way simple machines behave. It is not about advanced maths or a physics qualification — it is about reasoning through a picture of gears, levers, pulleys or ramps and working out what will happen. If you grasp a handful of core rules, this becomes one of the most winnable sections of the whole test. This guide sets out those rules in plain English, works through an example, and points you to some practice.

What Mechanical Comprehension tests

Each question gives you a short scenario, usually with a diagram, and asks you to predict an outcome: which way a gear turns, how much effort a lever needs, whether a load will lift, or how a ramp changes the force required. You are being tested on intuition about forces and motion, not calculation. A calculator is not allowed, but the arithmetic involved is deliberately simple. If you have not already, read our Defence Aptitude Assessment explained guide to see how this section fits into the wider test.

Gears

Gears crop up constantly, and a few reliable rules cover almost everything:

• Two meshed gears turn in opposite directions — if one turns clockwise, the gear touching it turns anticlockwise.
• A smaller gear turns faster than a larger one it is meshed with, because it has fewer teeth to move for each turn of the bigger gear.
• A big gear driving a small gear gives more speed at the small gear; a small gear driving a big gear gives more turning force (torque) but less speed.

In a chain of gears, count the meshes to work out direction: each meshed pair reverses, so gears alternate clockwise and anticlockwise along the line.

Levers and moments

A lever turns about a fixed point called the pivot (or fulcrum). The key idea is the moment, which is simply the turning effect of a force:

• Moment = force × distance from the pivot. A force further from the pivot has a bigger turning effect.
• A longer lever arm means less effort is needed to move the same load, because the effort acts over a greater distance from the pivot.

Levers come in three classes, defined by what sits in the middle:

1. First class — the pivot is in the middle, with effort and load on either side, like a see-saw or a pair of scissors.
2. Second class — the load is in the middle, between the pivot and the effort, like a wheelbarrow.
3. Third class — the effort is in the middle, between the pivot and the load, like a pair of tweezers or a fishing rod.

A worked example

Balance questions are common, and they all rely on the moment rule. Picture a see-saw with the pivot in the centre. On the left, a 10 kg weight sits 2 m from the pivot. On the right, a 20 kg weight sits 1 m from the pivot. Does it balance?

Work out the moment on each side using force × distance from the pivot:

• Left moment: 10 × 2 = 20.
• Right moment: 20 × 1 = 20.

The two moments are equal, so the see-saw balances. Notice the lighter weight sits further out and the heavier weight sits closer in — the extra distance makes up for the lighter load. That trade-off between force and distance is the heart of most lever questions.

Pulleys

Pulleys change how a lifting force is applied:

• A single fixed pulley only changes the direction of your pull — you pull down to lift a load up. It gives no mechanical advantage (a value of 1), so you still pull with a force equal to the load's weight.
• Adding more pulleys so that more rope sections support the load reduces the effort needed. Roughly, the more supporting rope sections there are, the less force you need — though you have to pull more rope through to move the load the same distance.

Inclined planes and ramps

A ramp lets you raise a load with less force than lifting it straight up. The rule is simple: a longer, gentler ramp needs less force, but you have to push the load over a greater distance. A short, steep ramp needs more force over a shorter distance. This is the same force-versus-distance trade-off you see with levers and pulleys.

Tips for the section

• Learn the basic rules above until they are second nature — gear direction, the moment rule, lever classes, and the effect of more rope sections or a gentler ramp.
• Sketch the set-up if a question is fiddly. Marking the pivot, or tracing which way each gear turns, prevents careless slips.
• Use everyday intuition. You already know a wheelbarrow makes heavy loads easier and a longer spanner loosens a tight bolt — trust that instinct and check it against the rules.

Put it into practice

Reading the rules is a start, but working through questions is what makes them stick. Try our mechanical comprehension practice to build confidence with real question styles, and see our how to prepare for the DAA guide for a full revision plan across every section. If electrical questions are next on your list, our companion guide covers circuits and current in the same plain-English style.

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