Using the Equations Sheet

In each Physics paper (Papers 5 and 6) you are given an equations sheet containing formulae that you do not need to memorise. However, many students struggle to use the sheet effectively — they pick the wrong equation, misidentify variables or forget to convert units. This lesson teaches you a systematic approach.

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What Is on the Equations Sheet?

The equations sheet contains additional Physics equations beyond those you must recall. These tend to involve more complex relationships. Examples include:

Equation on sheet	Context
p × V = constant (for a fixed mass of gas at constant temperature)	Gas pressure and volume
E = ½ × k × e²	Elastic potential energy
force on a conductor = magnetic flux density × current × length (F = BIl)	Magnetism / motor effect
V_p / V_s = n_p / n_s	Transformers
P = I²R	Power (alternative form)
P = V²/R	Power (alternative form)
(v)² = (u)² + 2as	Equations of motion
F = k × e	Hooke's Law
moment = force × distance	Moments
pressure = force ÷ area	Pressure
pressure (fluids) = height × density × g	Fluid pressure

Exam Tip: You receive the sheet automatically — you do not need to ask for it. Familiarise yourself with its layout before exam day so you can locate equations quickly.

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A Systematic Approach to Using the Sheet

flowchart TD
    A["Read the question"] --> B["Identify the <b>known</b> variables<br/>and the <b>unknown</b>"]
    B --> C["Look for an equation on<br/>the sheet that contains<br/>all your variables"]
    C --> D{"Is the equation<br/>in the right form?"}
    D -- Yes --> E["Substitute values"]
    D -- No --> F["Rearrange first,<br/>then substitute"]
    E --> G["Calculate and<br/>state the unit"]
    F --> G

Step-by-Step

1. Identify knowns and unknowns. Write down what you are given and what you need to find. Include units.
2. Select the equation. Scan the sheet for an equation containing your known variables and the unknown.
3. Check units. Before substituting, ensure all values are in the correct SI units.
4. Rearrange if necessary. Use the triangle method or algebraic rearrangement.
5. Substitute and calculate. Show each step clearly.
6. Write the answer with the correct unit.

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Common Mistakes When Using the Sheet

Mistake	Example	How to avoid it
Choosing the wrong equation	Using P = I × V when P = I²R is needed because V is not given	List your knowns first; the correct equation will contain those exact variables
Not converting units	Using cm instead of m in pressure = h × ρ × g	Always check units before substituting
Confusing similar symbols	Mixing up V (voltage) and v (velocity)	Read the context of the question carefully
Not rearranging	Trying to substitute into an equation that is not solved for the unknown	Rearrange before substituting
Forgetting the equation sheet exists	Attempting to derive equations from memory under pressure	In a physics paper, always glance at the sheet first

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Identifying Variables from the Question

Exam questions often use words rather than symbols. You need to translate:

Question phrase	Variable	Symbol	Unit
"the current flowing through"	Current	I	A
"the potential difference across"	Voltage / p.d.	V	V
"the extension of the spring"	Extension	e	m
"the spring constant"	Spring constant	k	N/m
"the stopping distance"	Distance	s	m
"the braking force"	Force	F	N
"the magnetic flux density"	B-field	B	T
"the number of turns on the primary coil"	Turns	n_p	—
"the height of the liquid"	Height	h	m
"the density of the liquid"	Density	ρ	kg/m³

Exam Tip: Practise translating question wording into symbols. This is the skill that lets you identify the correct equation quickly.

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Unit Conversion Before Substituting

The equations on the sheet assume SI units. Common conversions needed:

Given	Convert to	Method
cm → m	÷ 100	15 cm = 0.15 m
g → kg	÷ 1000	450 g = 0.45 kg
g/cm³ → kg/m³	× 1000	2.7 g/cm³ = 2700 kg/m³
kPa → Pa	× 1000	101 kPa = 101 000 Pa
cm² → m²	÷ 10 000	50 cm² = 0.005 m²
mA → A	÷ 1000	250 mA = 0.25 A
kW → W	× 1000	2.5 kW = 2500 W

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Worked Example

Question: A spring has a spring constant of 40 N/m. It is stretched and stores 0.80 J of elastic potential energy. Use the equation from the Physics Equations Sheet to calculate the extension of the spring.

Step	Working
Knowns	k = 40 N/m, E = 0.80 J, e = ?
Equation from sheet	E = ½ × k × e²
Rearrange	e² = 2E ÷ k
Substitute	e² = (2 × 0.80) ÷ 40 = 1.60 ÷ 40 = 0.04
Calculate	e = √0.04 = 0.20 m
Answer	Extension = 0.20 m

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When to Use Sheet Equations vs Recall Equations

A useful rule of thumb:

Situation	Action
The question says "use the Physics Equations Sheet"	Use the sheet
The question gives you the equation	Use the equation provided
The question says "calculate" or "use your knowledge" with no reference to the sheet	Use a recall equation
You are unsure	Check the sheet first — if the equation is there, use it

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Practising with the Equations Sheet