Study guide · Mechanical Comprehension Test
The OAR Mechanical Comprehension Test: Physics Concepts, Item Types and the Traps
The second-heaviest OAR subtest, in depth: the physical principles it really tests, how the diagram items are built, the high-yield concepts, and the traps to strike out — then practise it free on this site.
Subtest scope
What the Mechanical Comprehension Test Really Tests
The Mechanical Comprehension Test is the second-heaviest share of the OAR — about a third of this bank — so it earns your study time right after Math Skills, not last. It is not a physics exam. It never asks you to derive a formula or plug numbers into equations for a page; it shows you a picture — a lever, a set of gears, a pulley rig, a piston, a beam — and asks which way something turns, which setup takes less force, or which object tips first. The whole subtest rewards one habit: reason from the physical principle, then rule out every option that would violate it.
| Content area | What shows up | The principle behind it |
|---|---|---|
| Levers & the fulcrum | Which arm needs less force, where to place the pivot | A longer effort arm multiplies force |
| Pulleys | Fixed vs movable, block-and-tackle, which rig is easiest | Force drops with each supporting rope segment |
| Gears | Direction of turn, which shaft is faster, torque | Meshed gears turn opposite ways; big = slow & strong |
| Fluids & pressure | Hydraulic pistons, pressure in a closed system | Pressure = force ÷ area, transmitted equally |
| Gravity, weight & balance | Centre of gravity, stability, which object tips | A lower, wider base and a load over the base resist tipping |
| Simple machines & work | Inclined planes, wedges, screws, wheel-and-axle | Every machine trades force for distance — work is conserved |
Item taxonomy
Item Types and How the Diagrams Are Built
Read the answers before you study the diagram. The options tell you what the item is really testing — if the four choices are “clockwise / counter-clockwise / both / neither,” the question is about gear direction and nothing else, so you go straight to the rotation rule instead of measuring the picture. If the choices are “A takes less effort / B takes less effort / equal / cannot tell,” you are comparing mechanical advantage. Naming the item type first tells you which single principle to apply and lets you ignore everything in the diagram that does not bear on it.
The shapes a stem takes
| Item type | What the stem does | Your first elimination move |
|---|---|---|
| Direction of motion | Which way does gear/pulley/wheel X turn? | Apply “meshed gears reverse”; strike any same-direction option |
| Least / most effort | Which rig or lever needs less force? | Compare effort arms or rope segments; rule out the low-advantage setup |
| Speed vs force trade | Which shaft spins faster / has more torque? | Big gear = slow + strong; strike options that give you both |
| Balance & tipping | Which load balances the beam / which object tips? | Match force × distance on each side; rule out the unbalanced pick |
| Fluid / pressure | What happens to force or pressure in the system? | Use pressure = force ÷ area; kill options that ignore the area change |
Priorities
High-Yield Principles to Review
A short list of principles carries this subtest. Lock these in and you can reason your way to almost any answer without a formula. Drill them in this order — the first three appear the most and are the easiest to make automatic.
Meshed gears turn opposite ways
Two gears in contact rotate in opposite directions. Down a train, direction alternates — every other gear matches the driver. Count the meshes: odd number of gaps flips the direction, even keeps it. This single rule answers most gear items.
Big gear = slow and strong, small = fast and weak
The larger gear (more teeth) turns more slowly but delivers more torque; the smaller one spins faster with less force. You never get more speed and more force from the same pair — that trade-off is the whole point.
Pulleys: count the supporting rope segments
A fixed pulley only changes direction (no force saved). Each rope segment that actually supports the load divides the effort — two supporting segments halve the force, but you must pull twice the rope. More support = less force, more distance.
Levers: a longer effort arm multiplies force
Force × distance-from-fulcrum must balance across the pivot. Move the effort farther from the fulcrum, or the load closer, and the same push lifts more. Reading the two arm lengths settles almost every lever item.
Pressure = force ÷ area, and work is conserved
In a hydraulic system a small piston needs little force but moves far; the large piston pushes hard but barely moves. No machine — lever, pulley, ramp, hydraulic — multiplies work; it only trades force for distance.
Traps & tactics
Common Traps and Reasoning Shortcuts
The distractors here are engineered around the mistakes the test expects. The most common is the reversed-direction option: a gear or pulley answer that is correct in every way except it turns the wrong way. When “clockwise” and “counter-clockwise” both sit in the choices, that pair is the question — trace the meshes and one of them dies immediately.
Your reasoning shortcut is a physical sanity check. Before picking, ask whether the option obeys the principle: could this rig really let you lift more with less rope? Could both gears spin fast and push hard at once? An answer that quietly breaks conservation of work is the imposter, and it usually falls out before you do any counting.
Name the principle, strike every option that violates it, narrow to two, then commit the one the physics allows.
Worked examples
Worked Examples: Eliminate, Then Commit
Three short walk-throughs in the way you should actually work a mechanical item — answers first, name the principle, then eliminate. These are independent practice built to the public subtest outline, not real or retired test items.
Gears: kill the reversed-direction trap
“Three gears mesh in a row. The left gear turns clockwise — which way does the right gear turn?” Options: clockwise, counter-clockwise, cannot tell, it stops. Meshed gears reverse at each contact: left CW → middle CCW → right CW. Two meshes, an even number, so the far gear matches the driver. Strike “counter-clockwise” (the trap), “cannot tell,” and “stops” — commit clockwise.
Pulleys: count the support, not the rope you hold
“Which rig lifts a crate with the least effort?” Options: a single fixed pulley, one movable pulley, a two-movable-pulley block, or “all equal.” The fixed pulley only redirects the rope — no force saved — so strike it. “All equal” breaks the rule that support segments divide the load, so strike it too. More supporting segments means less force, so the two-movable block wins over the single movable. Commit the block-and-tackle.
Levers: read the two arm lengths
“A load sits on a bar over a fulcrum. To lift it with the least push, where do you apply force?” Options: close to the fulcrum, far from the fulcrum, directly over the load, or “position does not matter.” A longer effort arm multiplies force, so “close” and “over the load” (a zero-length arm) both raise the effort — strike them. “Does not matter” denies the lever principle outright. Commit “far from the fulcrum.”
Keep going
Where to Go Next
FAQ
Frequently Asked Questions
What is on the OAR mechanical comprehension test?
The Mechanical Comprehension Test covers applied physical principles: levers and fulcrums, pulleys and block-and-tackle rigs, gears and gear trains, fluids and pressure (hydraulics), gravity, weight and balance, and the simple machines — inclined planes, wedges, screws and the wheel-and-axle. Items are diagram-driven and ask which way something turns, which setup needs less force, or which object tips. It is intuition and reasoning, not formula-heavy calculation.
How do I study for mechanical comprehension?
Learn a short list of principles until they are automatic rather than memorising equations: meshed gears reverse direction, the larger gear is slower but stronger, pulleys divide force by their supporting rope segments, a longer lever arm multiplies force, and no machine multiplies total work. Then drill diagram items and, for each, name the principle and rule out every option that violates it. Study it right after the Math Skills Test, since it carries the second-heaviest weight.
Is the mechanical section on the OAR hard?
For candidates with hands-on or physics-class background it is often the most intuitive subtest; for others it feels hardest at first because the diagrams look busy. The content is shallow — a handful of principles — and the difficulty is reading the picture quickly and avoiding the reversed-direction and speed-versus-force traps. Reasoning from the principle and eliminating impossible options makes it very manageable.
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