### Introduction

Formulae are used frequently in almost all aspects of engineering in order to relate a physical quantity to one or more others. Many well-known physical laws are described using formulae. For example, you may have already seen Ohm’s law, $v=iR$ , or Newton’s second law of motion, $F=ma$ .

In this Section we describe the process of evaluating a formula, explain what is meant by the
**
subject
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of a formula, and show how a formula is rearranged or transposed. These are basic skills required in all aspects of engineering.

#### Prerequisites

- be able to add, subtract, multiply and divide algebraic fractions

#### Learning Outcomes

- transpose a formula

#### Contents

1 Using formulae and substitution2 Rearranging a formula

3 Engineering Example 2

3.1 Heat flow in an insulated metal plate