Physics Calculations: A Four-Step Method That Protects Your Marks (With Worked Examples)

 

By Zaheer | Updated 14 October 2026

Disclosure: I created the Physics Formula Lab & Simulator mentioned at the end and sell it on Gumroad. Everything in this post is free to use without it. This is independent study material, not affiliated with Cambridge or any exam board.

Physics calculations are some of the most reliable marks on the paper, and some of the easiest to throw away. Students often know the formula and still lose marks because of a missing unit conversion, a wrong power, or an answer with no unit. This post gives you a four-step method that works for IGCSE Physics (0625) and O Level Physics (5054), five worked examples that show where the traps are, and four questions to try yourself.

The four-step method

  1. Write the formula first. Even if the question looks easy, writing the formula usually earns a mark by itself.
  2. Convert units before you substitute. Change everything to standard units (metres, kilograms, seconds) at this stage, not at the end.
  3. Substitute with units, then calculate. Show the numbers in the formula so the examiner can award method marks even if your final answer is wrong.
  4. State the answer with its unit. Give a sensible number of significant figures, usually two or three, matching the data in the question.

Unit conversions that cause most of the trouble

From To What to do
grams (g)kilograms (kg)divide by 1000
cm2m2multiply by 10−4
cm3m3multiply by 10−6
minutessecondsmultiply by 60
hourssecondsmultiply by 3600
kilowatts (kW)watts (W)multiply by 1000

Squared and cubed units are the usual trap: 1 cm2 is not 0.01 m2. It is 0.0001 m2, because both sides of the square are shrunk by 100.

Five worked examples

1. Speed (no conversion needed)

A car travels 150 m in 12 s. Find its average speed.

  • Formula: speed = distance ÷ time
  • Substitute: speed = 150 ÷ 12
  • Answer: 12.5 m/s

2. Density (convert first)

A block has a mass of 540 g and a volume of 200 cm3. Find its density in kg/m3.

  • Formula: density = mass ÷ volume
  • Convert: 540 g = 0.54 kg and 200 cm3 = 2.0 × 10−4 m3
  • Substitute: density = 0.54 ÷ (2.0 × 10−4)
  • Answer: 2700 kg/m3

3. Pressure (area in cm2)

A box pushes down on a table with a force of 120 N. The area in contact is 400 cm2. Find the pressure.

  • Formula: pressure = force ÷ area
  • Convert: 400 cm2 = 400 × 10−4 = 0.040 m2
  • Substitute: pressure = 120 ÷ 0.040
  • Answer: 3000 Pa

4. Energy (kW and minutes)

A 2.0 kW heater is switched on for 30 minutes. How much energy does it transfer?

  • Formula: energy = power × time
  • Convert: 2.0 kW = 2000 W and 30 min = 1800 s
  • Substitute: energy = 2000 × 1800
  • Answer: 3,600,000 J (3.6 MJ)

5. Kinetic energy (square the speed only)

A trolley of mass 3.0 kg moves at 4.0 m/s. Find its kinetic energy.

  • Formula: KE = ½ × mass × speed2
  • Substitute: KE = ½ × 3.0 × (4.0)2 = ½ × 3.0 × 16
  • Answer: 24 J

Five mistakes that cost marks

  1. Skipping the formula. A bare number with no working can score zero if it is wrong.
  2. Converting too late, or not at all. Using 400 instead of 0.040 in the pressure example gives an answer 10,000 times too small.
  3. Squaring the wrong thing. In kinetic energy only the speed is squared, not mass × speed.
  4. Leaving out the unit, or using the wrong one. "3000" and "3000 N" both lose the mark. The unit here is Pa.
  5. Rounding too early. Keep full calculator values until the last line, then round once.

Try these four questions

Use all four steps, then check your answers.

  1. A runner covers 90 m in 7.5 s. Calculate the average speed.
  2. A metal block has a mass of 2400 g and a volume of 3000 cm3. Calculate its density in kg/m3.
  3. A force of 90 N acts on an area of 150 cm2. Calculate the pressure.
  4. A 60 W lamp is on for 2.0 hours. Calculate the energy transferred in joules.

Answers

  1. Speed = 90 ÷ 7.5 = 12 m/s.
  2. Mass = 2.4 kg and volume = 3.0 × 10−3 m3. Density = 2.4 ÷ (3.0 × 10−3) = 800 kg/m3.
  3. Area = 150 × 10−4 = 0.015 m2. Pressure = 90 ÷ 0.015 = 6000 Pa.
  4. Time = 2.0 × 3600 = 7200 s. Energy = 60 × 7200 = 432,000 J (4.3 × 105 J to 2 significant figures).

Quick checklist

  • I write the formula before any numbers.
  • I convert g, cm2, cm3, minutes, hours and kW before substituting.
  • I show the substitution so I can earn method marks.
  • I finish with the right unit and a sensible number of significant figures.
  • I round only once, at the end.

Practise more

My Physics Formula Lab & Simulator is an interactive tool built to practise using formulas for IGCSE Physics 0625 and O Level Physics 5054, so you understand them and don't just memorise them: Physics Formula Lab & Simulator.

Always check the official syllabus and specimen papers for the latest requirements, including which formulas are provided in the exam. No exam result is guaranteed.

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