# Definition:Speed

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## Contents

## Definition

The **speed** of a body is a measure of the magnitude of its velocity, taking no account of its direction.

It is, therefore, a scalar quantity.

### Symbol

The usual symbol used to denote the **speed** of a body is $v$.

## Examples

### Speed of Sound in Air

The speed of sound in air at ground level on Earth is approximately $330 \, \mathrm {m s}^{-1}$.

### Speed of Light

The **speed of light (in a vacuum)** is a physical constant.

Information cannot travel faster.

It is usually denoted $c$, and its value is given as:

- $c = 299 \, 792 \, 458 \text { m s}^{-1}$

**exactly**.

The metre is in fact *defined* in terms of the **speed of light** and the definition of the second.

## Also known as

Some authors erroneously or carelessly refer to the **speed** of a body as its velocity.

But this is technically wrong if the author does not specify its direction as well as its magnitude.

## Sources

- 1966: Isaac Asimov:
*Understanding Physics*... (previous) ... (next): $\text {I}$: Motion, Sound and Heat: Chapter $3$: The Laws of Motion: Forces and Vectors - 1968: Murray R. Spiegel:
*Mathematical Handbook of Formulas and Tables*... (previous) ... (next): $\S 22$: Vectors and Scalars - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next): Entry:**speed** - 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next): Entry:**speed**