Many lost marks in numerical questions come from units and substitution, not from ideas. Use this checklist after you finish any calculation in maths, physics, chemistry or geography.
It takes about a minute per question, and it is worth practising on easy questions until it is automatic.
The checklist
| Step | Check | Tick |
|---|---|---|
| 1 | I wrote the formula in symbols before using numbers | |
| 2 | I listed each quantity with its unit, including what the question did not label clearly | |
| 3 | All units are compatible (hours with hours, cm with cm). If not, I converted first | |
| 4 | I substituted into a second line and did the arithmetic step by step | |
| 5 | My answer has a unit and the unit matches the formula | |
| 6 | I checked the size and sign with a rough estimate | |
| 7 | I rounded only at the end, as the question asked |
Filled example 1: speed
A car travels 150 km in 1 hour 30 minutes. Find its average speed in km/h, then in m/s.
- Formula: speed = distance ÷ time.
- Quantities: distance = 150 km; time = 1 h 30 min.
- Compatible units: 30 minutes = 0.5 h, so time = 1.5 h.
- Substitute: speed = 150 ÷ 1.5 = 100 km/h.
- Convert to m/s: 100 km = 100 000 m and 1 h = 3600 s, so 100 000 ÷ 3600 = 27.78 m/s (2 decimal places).
- Reasonableness: 100 km/h is about 28 m/s, so 27.78 is sensible.
The common slip: writing 1 h 30 min as 1.3 h gives 150 ÷ 1.3 = 115.4 km/h, which is wrong. Minutes are not hundredths of an hour, so 30 minutes is 0.5 h.
Filled example 2: force
A trolley of mass 1200 kg accelerates at 2.5 m/s². Find the resultant force.
- Formula: F = m × a.
- Quantities: m = 1200 kg; a = 2.5 m/s².
- Units compatible: kg and m/s² give newtons.
- Substitute: F = 1200 × 2.5 = 3000.
- Answer with unit: F = 3000 N.
- Reasonableness: 1000 × 2.5 is 2500, and 200 × 2.5 is 500, so 3000 is correct.
Common unit traps
- Mixing time units: minutes or seconds with hours in the same formula.
- Area and volume conversions: 1 m² = 10 000 cm², not 100 cm².
- Mass in grams when the formula expects kilograms.
- Leaving out the unit on the final line.
- Rounding too early, which shifts the final digit.
The non-calculator working trainer helps you practise exact steps and independent checking without a calculator. To record the slips you find, use the mistake log template, and see more on the resources page.
Units and substitution slips are a habit problem more than a knowledge problem. Our teachers watch working live in online one-to-one Physics tuition and other subjects, so the habit can be corrected as it happens.