Lesson: Chapter 4 — Waves and their applications

4.1.3. Describing a wave 5 of 5

Wave speed

In one period T the wave travels a distance equal to one wavelength λ. That is enough to find its speed.

Speed is distance divided by time, so v = λ / T. And since 1 / T is the frequency, the same thing can be written v = f λ.

Wave speed

v = f λ

v in m s^-1, f in Hz, λ in m

An example

Problem

A wave on a rope has a frequency of 5 Hz and a wavelength of 2 m. How fast does the wave travel?

Solution

v = f λ

= 5 Hz × 2 m

= 10 m s^-1

Four quantities, two links

f = 1 / T joins frequency and period. v = f λ joins speed, frequency and wavelength. Amplitude is the odd one out: it does not appear in either, because how far the particles move does not change how fast the wave travels.