AQA A-Level Physics: Fundamental and Derived Units in Physics — mark scheme explained

The question

A car travels at a speed of 80 km/h. Convert this speed to metres per second (m/s). [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Identify the conversion factor between kilometres and metres.

• S2

  1 kilometre = 1000 metres

• S3

  Step 2: Identify the conversion factor between hours and seconds.

• S4

  1 hour = 3600 seconds

• S5

  Step 3: Convert the speed from km/h to m/s using these factors.

• S6

  80 km/h × (1000 m/km) ÷ (3600 s/h) = 22.22 m/s

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Identify the conversion factor between kilometres and metres.

2. S2

   1 kilometre = 1000 metres

3. S3

   Step 2: Identify the conversion factor between hours and seconds.

4. S4

   1 hour = 3600 seconds

5. S5

   Step 3: Convert the speed from km/h to m/s using these factors.

6. S6

   80 km/h × (1000 m/km) ÷ (3600 s/h) = 22.22 m/s

7. Final answer: 22.22 m/s

Common mistakes

• Confusing the base unit for mass with grams instead of kilograms. — Always remember that the SI base unit for mass is the kilogram (kg), not grams (g).
• Forgetting to convert units before performing calculations. — Always check and convert units to a common standard (e.g., metres, kilograms, seconds) before performing any calculations.
• Using the wrong conversion factor for unit conversions. — Memorize and use the correct conversion factors. For example, 1 km = 1000 m, 1 hour = 3600 seconds, 1 eV = 1.602 × 10 -19 J.
• Misusing SI prefixes in calculations. — Always double-check the correct power of ten for each SI prefix. For example, 1 μA = 10 -6 A, 1 kN = 10 3 N.
• Failing to use standard form for very large or small numbers. — Use standard form (scientific notation) to express very large or small numbers. For example, 300,000,000 m/s should be written as 3 × 10 8 m/s.
• Confusing the derived unit for energy with power. — Remember that energy is measured in joules (J) and power is measured in watts (W). Energy = Power × Time. For example, 1 kWh = 3.6 × 10 6 J.

Where the marks go

• Full worked solution (all marking points)4 marks

Related questions

• AQA A-Level Physics: Errors and Uncertainties in Measurements — mark scheme explained →
• AQA A-Level Physics: Properties of Quarks and Antiquarks — mark scheme explained →