Most lost marks on a mechanical reasoning test are not a gap in physics. They are one of a handful of slips, repeated. Learn the seven below and a one-line check for each.
1. Turning the Ratio Upside Down
A 12-tooth gear at 90 rpm drives a 36-tooth gear. Speed out = 90 x 12 / 36 = 30 rpm. The slip gives 90 x 36 / 12 = 270 rpm.
Check: the driven gear has more teeth, so it must turn slower. Any answer above 90 rpm fails.
2. Measuring From the Wrong Place
On a lever, the distance that counts is from the pivot to the force, not from the end of the bar or from one load to the other. A 2 m bar with its pivot 0.5 m from one end has arms of 0.5 m and 1.5 m, not 2 m and 0.
Check: mark the pivot first and write each arm beside it before any multiplication.
3. Counting Wheels Instead of Rope Sections
The mechanical advantage of a block and tackle is the number of rope sections supporting the moving block, not the number of wheels. A moving block with a wheel and a hook on a single loop of rope has two sections, whatever else is drawn.
Check: trace the rope with a finger. Count only the sections that pull on the moving block. See pulleys and mechanical advantage.
4. Forgetting That Meshed Gears Reverse
In a chain of three gears where each meshes with the next, the first and third turn the same way and the middle one the opposite way. Compound trains with a shared shaft add one more twist: gears on the same shaft turn together.
Check: write clockwise or anticlockwise under each gear as you go, not at the end.
5. Mixing Units in Pressure Questions
A force of 50 N on a piston of 5 cm² gives 10 N per cm². Converted properly, 5 cm² is 0.0005 m², so the pressure is 50 / 0.0005 = 100,000 Pa. Dividing 50 by 5 and calling the result 10 Pa is out by a factor of 10,000.
Check: write the unit on every number, and convert before you divide.
6. Forgetting the Weight of the Bar, or Counting It Twice
If a question says the plank is light, ignore its weight. If it says the plank is uniform and gives its weight, add that weight as one force at the middle. A 3 m uniform plank weighing 60 N acts as 60 N at 1.5 m from either end.
Check: reread the first line of the question for the word "light" or "uniform" before you start.
7. Confusing Series and Parallel Springs
Two identical springs side by side share the load, so each stretches half as much as one spring alone. Two end to end each carry the whole load, so the pair stretches twice as far as one. The extension, not the length, is what adds in series.
Check: ask whether the load passes through both springs one after the other (series) or splits between them (parallel). Rules and numbers are in pressure, hydraulics and springs.
Slips That Come From the Wording
Some marks are lost to the way a question is phrased rather than to the physics. Read for these four words before you start:
- Light or uniform: whether the bar's own weight counts (see mistake 6).
- Extension or length: a spring's increase over its natural length, or its whole length.
- Slowly or constant temperature: the signal that the simple gas rule applies.
- Ignore friction: use the ideal rule, and do not add losses the question did not ask for.
Also check which quantity the question asks for. A single set of facts can lead to the speed of a gear, its direction, or its torque. Underline the one asked for before you calculate anything.
Keep a Mistakes Log
A log turns random misses into a short list you can fix. One line per miss is enough: the topic, the slip, and the check that would have caught it.
- Gears: flipped the ratio. Check: more teeth means slower.
- Lever: measured from the end of the bar. Check: mark the pivot first.
- Pressure: used cm² with m². Check: convert before dividing.
After a week, one or two of these will repeat. Those are the ones to rehearse. A slip you have named is much less likely to catch you out again.
A Habit That Catches All Seven
Sense-check every answer for size and direction before you pick it. The four-step routine in how to solve mechanical reasoning questions builds that in. Then use the JobCannon mechanical reasoning test to see which of the seven you make, and keep a one-line log of each miss.