A mechanical wave carries energy through a medium by vibrating its particles. Learn the two types, transverse (particles perpendicular, with crests and troughs) and longitudinal (particles parallel, with compressions and rarefactions), how they differ, and examples like sound and water waves.
What a mechanical wave is
A mechanical wave is a wave that carries energy through a medium — a material such as water, air, or a rope — by making the particles of that medium vibrate. The particles pass the energy along without traveling with it, so a mechanical wave transfers energy, not matter. Because it needs particles to move, a mechanical wave cannot travel through empty space. If you are new to waves, start with the introduction to waves.
Transverse waves
In a transverse wave, the particles of the medium vibrate perpendicular (at a right angle) to the direction the wave travels. The high points are crests and the low points are troughs. The wavelength is the distance between two crests, and the amplitude is the height from the rest position to a crest.
A mechanical wave shown as a transverse wave: particles move up and down while the wave travels sideways.
A wave on a rope or ripples on a water surface are transverse.
Longitudinal waves
In a longitudinal wave, the particles vibrate parallel to the direction the wave travels — back and forth along the same line. Where the particles bunch together is a compression; where they spread apart is a rarefaction.
A longitudinal wave: particles move back and forth, creating compressions and rarefactions.
Sound waves are the most common longitudinal waves.
Transverse vs longitudinal
The one difference to remember: in a transverse wave the particles move across the wave's path, while in a longitudinal wave they move along it. Both still carry energy through a medium, and both can add together where they meet, which is called superposition of waves.
Examples of mechanical waves
Sound — a longitudinal wave through air, water, or solids.
Water waves and waves on a string — transverse.
Seismic waves from earthquakes — both types (P-waves are longitudinal, S-waves are transverse).