OAR Mechanical Comprehension: Fluids, Gases and Circuits

What's published, and what isn't

Source: NMOTC ASTB-E pages. There's no calculator on the ASTB-E; relevant formulas are provided. Our length and clock are our own.

How it counts

The MCT is one of the three parts of the OAR (20–80, in one-point steps), the score that predicts OCS academic performance. Full-battery takers also receive AQR, PFAR and FOFAR stanines from 1 to 9. These formulas are proprietary and compensatory, and NMOTC publishes no OAR passing score: your program sets its own minimum. Scores in our app are estimates.

The adaptive rules apply: answer too few and a penalty may follow, and random guessing near the end is advised against. Your latest attempt replaces earlier scores, even higher ones.

What it tests

Fluids and gases (pressure, flow, buoyancy), electricity (series and parallel circuits) and machines (levers, pulleys, gears). A useful habit: ask which way a quantity moves when something changes, because a clear rule beats a long calculation.

The technique: direction first, size second

For each question, decide the direction of change first: up or down, faster or slower, brighter or dimmer. Get it from the governing rule, then attach a number. A direction check alone often removes three options, and it protects you on a test where you can't come back.

Three worked examples

1. Flow in a narrowing pipe. Water flows through a level pipe that narrows from 4 cm² to 2 cm² in cross-section. In the narrow part, the water is: A. slower, at higher pressure B. faster, at higher pressure C. faster, at lower pressure D. the same speed, at lower pressure E. slower, at lower pressure. Answer: C. The same volume passes each second, so half the area means twice the speed. By Bernoulli's principle, where a fluid speeds up in a level pipe, its pressure drops.

2. Squeezing a gas. A sealed cylinder holds gas at 30 psi absolute. A piston slowly pushes it to one-third of its volume at constant temperature. What is the new pressure? A. 10 psi B. 33 psi C. 60 psi D. 90 psi E. 270 psi. Answer: D. At constant temperature, pressure times volume stays constant (Boyle's law): one-third the volume, three times the pressure, so 30 × 3 = 90 psi. A divides instead of multiplying; E multiplies by 9.

3. Series or parallel. Two identical bulbs run from a 12-volt battery, first in series, then in parallel. In which arrangement is each bulb brighter? Answer: parallel. Each bulb then gets the full 12 volts; in series they split it, 6 volts each. And if one bulb fails, a series circuit goes dark, while in parallel the other bulb stays lit.

Traps

• Narrow pipe, high pressure. Intuition says squeezed water pushes harder. In steady flow, the faster section has the lower pressure.
• Gauge vs absolute pressure. Gas-law ratios work on absolute pressure.
• Series vs parallel. Adding a bulb in series dims the others; adding one in parallel doesn't, with an ideal battery.
• Mixed-up machine rules. More teeth means a slower gear; more supporting rope segments mean less pull.
• Random answers late in the section. NMOTC advises against them on adaptive subtests.

Pacing without a published clock

The time limit isn't published, so build speed through recall: once the rules are automatic, most questions are a direction check plus one step. Our timed set is 30 questions in 15 minutes, 30 seconds each, a brisk pace that rewards knowing the rules cold.

Practice it here

Our MCT practice doesn't adapt the way APEX does; it's fixed-length, and the timing is ours. Setups are described in words, and every explanation names the principle. Start in practice mode, then the timed set, answering each question as if you can't return. The Army's SIFT has its own adaptive mechanical test; see SIFT Mechanical Comprehension for gears, levers and pulleys.

How to solve an OAR mechanical question

1. Identify the system: fluid, gas, circuit or machine.
2. Recall the governing rule, such as area times speed staying constant, or pressure times volume staying constant.
3. Decide the direction of change before using any numbers.
4. Cross out options that move the wrong way.
5. Attach the number with one multiplication or division.
6. Confirm the answer makes physical sense, then commit; there's no going back.

FAQ