Units, prefixes and standard form
Convert prefixes, squared and cubed units. Write numbers in standard form.
Physics equation practice
Choose an equation, rearrange it and convert the units before calculating.
Practise
Choose an equation: energyA heater transfers energy at a constant power of 700 W for 6 min. Calculate the energy transferred in J. Give your answer to 3 significant figures.
Work through the calculation
Check intermediate steps, then use your unrounded working for the final answer. Each numerical check uses 3 significant figures. These steps provide support; the main answer is checked separately.
Step 1
Convert the heating time to seconds. Give your answer to 3 significant figures.
Show this step
Multiply the time in minutes by 60.
Answer: 360 s (3 significant figures).
Step 2
Now calculate the final answer to the main question.
Show this step
Power is energy transferred per second. Convert the time to seconds, then find E = Pt.
Answer: 2.52 × 10^5 J (3 significant figures).
Treat the given values as exact unless an uncertainty is stated.
Hint
Power is energy transferred per second. Convert the time to seconds, then find E = Pt.
Worked solution
- Equation: P = E / t.
- t = 6 × 60 = 360 s.
- E = Pt = 700 × 360 J.
- Answer: 2.52 × 105 J (3 significant figures).
Question type: Mixed practice
Report a problem
Choose an equation before calculating
Look for an equation connecting the quantity you need with quantities you know. Rearrange it before putting in numbers, then put the units in a compatible form.
For a resistor with potential difference 6.0 V and current 20 mA, use V = IR. Rearrange to R = V / I. Since 20 mA = 0.020 A, R = 6.0 / 0.020 = 300 Ω.
Units and powers
Use seconds with watts to find energy in joules. Use kilograms in F = ma and Eₖ = ½mv². For density in kg m⁻³, convert cm³ to m³ using 1 cm³ = 10⁻⁶ m³.
The square in kinetic energy applies to speed. Doubling speed multiplies kinetic energy by four for the same mass.
Some questions include a quantity you do not need. Average speed requires distance and time; the vehicle's mass does not enter that calculation.
What this practice includes
Resistance/current, energy/power/time, resultant force/mass, density, average speed and kinetic energy. Some questions ask you to choose an equation; others supply it so you can practise rearrangement.
Use 3 significant figures for these calculations. The given values are treated as exact for practice. More precise answers that round to the same value are accepted; trailing zeros need not be entered.
A-level relevance
AQA 7408: 3.4.1.3, 3.4.1.5, 3.4.1.7–8, 3.4.2.1 (selected); AQA 7408: 3.5.1.1 (selected); OCR A H556: 3.1.1, 3.2.1, 3.2.4(a), 3.3.2–3, 4.2 (selected). Tasks cover the selected skills listed above.
AQA mechanics and materials specification · AQA electricity specification · OCR A specification