Definition:Speed

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): speed
• 2014: Christopher Clapham and James Nicholson: The Concise Oxford Dictionary of Mathematics (5th ed.) ... (previous) ... (next): speed