Mechanical reasoning tests draw on a defined set of physics topics, so knowing how each one works tells you exactly what to revise. This guide covers the four that carry most of the marks.
Practise the real thing: every topic below appears in our free mechanical reasoning test, with a worked solution for every question. No sign-up.
How gears work
When two gears are meshed together, they turn in opposite directions, and the one with fewer teeth turns faster. Gear speed is inversely proportional to the number of teeth: a gear with twice as many teeth as its neighbour turns at half the speed.
That single rule answers most gear questions. Take a worked example: if a small cog with 5 teeth completes 40 revolutions per second and it drives a larger cog with four times as many teeth, the larger cog turns four times more slowly, so it completes 10 revolutions per second. You do not need to count anything beyond the ratio of the teeth.
For a chain or belt of several gears, direction alternates along the line, meshed neighbours turn opposite ways, while two gears joined on the same axle turn together at the same speed. Work along the train one pair at a time and the answer follows.
How levers and moments work
A lever balances when the turning effect on each side of the pivot is equal, and the turning effect, the moment, is the force multiplied by its distance from the pivot. So a small force far from the pivot can balance a large force close to it.
To answer a lever question, multiply the load by its distance from the fulcrum on one side, then find the force or distance that produces the same product on the other. If a 10 kg weight sits two metres from the pivot, the moment is 20, so an effort one metre from the pivot on the other side must supply 20 units of turning effect, meaning 20 kg. Moving the effort further from the pivot reduces the force needed, which is exactly why a longer spanner loosens a stubborn bolt.
The same principle underlies the different classes of lever, where the relative positions of effort, load and fulcrum change, but the balance of moments always decides the answer.
How pulleys work
A pulley system shares the weight of a load across several sections of rope, so the more rope sections directly support the load, the less force you need to lift it. With a single fixed pulley you gain only a change of direction, but each additional supporting rope section divides the effort further.
The rule is simple: count the rope sections supporting the load, and divide the load by that number to find the effort required. In a worked example, if two sections of rope support a weight, the force needed is the weight divided by two; a 120 kg load supported by two rope sections needs 60 kg of effort to lift. Four supporting sections would cut it to a quarter.
The trade-off, worth remembering for questions that ask about it, is distance: halving the effort means pulling twice as much rope through, because the work done stays the same.
How circuit and magnet questions work
For circuits, trace the path of the current and remember that a series circuit has one loop while a parallel circuit offers several. In a series circuit, breaking the loop anywhere, an open switch or a failed bulb, stops everything; in a parallel circuit each branch is independent, so one break need not disable the rest. Follow the wire from the power source, through each switch and component, and back, to see what stays powered.
For magnets, the rule is that opposite poles attract and like poles repel. In a typical question showing several pairs of magnets, you identify which pairs have a north facing a south, and those are the ones that attract, while the pairs with two norths or two souths facing each other repel. Applying that pole rule to each pair in turn gives the answer directly.
More on mechanical reasoning tests
Go back to the full mechanical reasoning tests guide, or read about the Bennett Mechanical Comprehension Test.
