# Speed of Medium Compared to Wave Speed

Do the particles in a medium oscillate about their equilibrium position at the same speed as the wave moves through the medium? Is it the same for both transverse and longitudinal waves?

## 2 Answers

No. The speed of oscillation increases with amplitude and frequency. This is true for both transverse and longitudinal waves.

There is another velocity: the speeds in random directions of molecules in a gas. These are (on average) larger than the speed of sound in that gas, but of the same order of magnitude.

Suppose a wave $y(x,t)=A\sin\left (2\pi f t - \frac{2\pi x}{\lambda}\right )$ where $y$ is the displacement of a particle from its equilibrium position which is a distance $x$ from an origin at a time $t$.
$f$ is the frequency of the wave and $\lambda$ is the wavelength.

The particle moves with simple harmonic motion, amplitude $A$ and frequency $f$ so the maximum speed of the particle is $2\pi f A$ and its minimum speed is zero.
The wave moves at a constant speed of $f\lambda$.

This is true for both longitudinal and transverse waves.