Calculation Techniques for Physics

Calculations make up a significant proportion of the marks on both Edexcel GCSE Physics (1PH0) papers — approximately 30% of all marks require mathematical skills. This lesson covers the essential techniques you need to maximise your marks on calculation questions.

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The Four-Step Method

Every calculation in physics should follow this method:

Step	Action	Why It Matters
1	Write the equation	Shows the examiner which equation you are using (earns a mark)
2	Substitute the values	Shows you can identify the correct quantities
3	Calculate	Perform the arithmetic
4	State the answer with units	A missing unit can lose you a mark

Worked Example

Calculate the weight of an object with a mass of 75 kg. (gravitational field strength g = 9.8 N/kg)

Step 1: W = mg

Step 2: W = 75 × 9.8

Step 3: W = 735

Step 4: W = 735 N

Exam Tip: Even if the question does not say "show your working", always show it. If your final answer is wrong but your method is correct, you can still earn marks. A bare wrong answer earns zero.

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The Equation Triangle Method

Many GCSE physics equations involve three variables. You can use a formula triangle to help you rearrange equations quickly.

How It Works

Write the three variables in a triangle. The variable on top is divided by either of the bottom two. The bottom two are multiplied together.

Example: v = s / t (speed = distance ÷ time)

    ┌───┐
    │ s │
    ├───┤
    │v│t│
    └───┘

• To find s: cover s → v × t (multiply the bottom two)
• To find v: cover v → s ÷ t (divide top by remaining bottom)
• To find t: cover t → s ÷ v (divide top by remaining bottom)

Exam Tip: Formula triangles are a useful tool, but for Higher tier you should also practise rearranging equations algebraically. Some exam questions specifically ask you to rearrange an equation before substituting.

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Rearranging Equations

For Higher tier especially, you need to rearrange equations algebraically. The key rule: whatever you do to one side, you must do to the other.

Example 1: Rearrange V = IR to find R

V = IR

Divide both sides by I:

V / I = R

So: R = V / I

Example 2: Rearrange KE = ½mv² to find v

KE = ½mv²

Multiply both sides by 2:

2 × KE = mv²

Divide both sides by m:

2KE / m = v²

Take the square root of both sides:

v = √(2KE / m)

Exam Tip: When rearranging, think of it as "undoing" operations in reverse order. If the equation multiplies by ½ then by m then squares v, you undo by: ×2, ÷m, then √.

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The Physics Equation Sheet

In the exam, you will receive an equation sheet. It is critical to know which equations are on the sheet and which you must memorise.

Situation	What to Do
Equation is on the sheet	You still need to know what it means and when to use it
Equation is NOT on the sheet	You must memorise it — it will not be provided

Lessons 5 and 6 give the complete breakdown. For now, the key message is: do not rely on the equation sheet alone. You must be fluent in using ALL equations.

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Unit Conversions

Many exam questions require you to convert units before substituting into an equation. Getting the units wrong is one of the most common reasons for losing marks.

Essential Conversions

From	To	Method	Example
km/h	m/s	Divide by 3.6	72 km/h = 72 ÷ 3.6 = 20 m/s
m/s	km/h	Multiply by 3.6	20 m/s = 20 × 3.6 = 72 km/h
kW	W	Multiply by 1000	2.5 kW = 2500 W
W	kW	Divide by 1000	500 W = 0.5 kW
kWh	J	Multiply by 3,600,000	2 kWh = 7,200,000 J
J	kWh	Divide by 3,600,000	3,600,000 J = 1 kWh
minutes	seconds	Multiply by 60	5 min = 300 s
hours	seconds	Multiply by 3600	2 hours = 7200 s
km	m	Multiply by 1000	3.5 km = 3500 m
cm	m	Divide by 100	50 cm = 0.5 m
g	kg	Divide by 1000	250 g = 0.25 kg

Exam Tip: Always check the units given in the question against the units required by the equation. If the question gives speed in km/h but the equation requires m/s, you MUST convert before substituting. This is a very common source of errors.

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Significant Figures

The general rule for GCSE physics: give your answer to the same number of significant figures as the data in the question, or to 2 or 3 significant figures if not specified.

Rules for Significant Figures

1. All non-zero digits are significant: 456 has 3 s.f.
2. Zeros between non-zero digits are significant: 305 has 3 s.f.
3. Leading zeros are NOT significant: 0.0045 has 2 s.f.
4. Trailing zeros after a decimal point ARE significant: 2.50 has 3 s.f.

Example

A student calculates a speed as 4.285714 m/s. The data in the question is given to 2 significant figures. Give the answer to an appropriate number of significant figures.

Answer: 4.3 m/s (2 s.f.)