A wave is a disturbance that helps to transfer energy from one place to another. Mechanical waves are the waves that necessarily require a medium to travel from one position to the other. This implies that a mechanical wave cannot travel through the vacuum. When a disturbance is introduced into a medium, it causes the particles of the medium to oscillate periodically about their mean positions. The motion of one oscillating particle gets transferred to its neighbouring particle, and the process goes on and on till the energy fades away. This process of energy transfer allows the disturbance to move from one place to the other. Hence, a mechanical wave is said to be travelling.
Types of Mechanical Waves
Depending on the direction of wave propagation, mechanical waves can be classified into two broad categories, namely, transverse waves and longitudinal waves.
1. Transverse Waves
A mechanical wave is said to be transverse if the particles of the medium tend to oscillate in a direction perpendicular to the direction of propagation of the wave. This implies that the particles of the medium vibrate up and down. In other words, the particle motion and the wave motion of a transverse wave are aligned perpendicular to each other. The maximum displacement formed by a transverse wave in the upward direction is known as a crest, whereas the maximum displacement formed by a transverse wave in the downward direction is known as a trough. They are also known as t-waves.
2. Longitudinal Waves
A mechanical wave is said to be longitudinal if the particles of the medium tend to oscillate in a direction parallel to the direction of propagation of the wave. This implies that the particles of the medium vibrate to and fro. Longitudinal waves travel in the form of compression and rarefaction. Compressions demonstrate the region of maximum pressure and density, while rarefaction is the region of minimum pressure and density. Longitudinal waves are also known as i-waves.
Examples of Mechanical Waves
1. Sound Waves
Sound waves are a prominent example of mechanical waves. This is because a sound wave necessarily requires a medium to propagate from one location to the other. In other words, the sound is incapable of travelling through a vacuum. Ultrasonic and infrasonic sound waves are inaudible to humans, but they are also an example of mechanical waves.
2. Water Waves
Due to the action of the gravitational force of the moon, the earth, and the sun, the water present in the seas or oceans receives a pull force, leading to the formation of a wave. The crests and troughs of such waves are easily visible. Hence, the ocean and sea waves are yet another example of transverse mechanical waves in real life.
3. Spring Waves
When a push or a pull force is applied to one end of a spring or a slinky, keeping its opposite end stable, its particles tend to vibrate back and forth in a direction parallel to the movement of the spring. The compressions and rarefactions formed by a deformed spring can be easily observed. Hence, the waves produced by a spring or a slinky are known as longitudinal mechanical waves.
4. Stadium Waves
A stadium wave or the Mexican wave is formed when a group of people pull their arms up rhythmically one after another. The audience present in the stadium acts as a medium for the wave to travel from one location to the other. Hence, the stadium wave is a classic example of mechanical waves.
5. Jump Rope Waves
Jump rope is common exercise equipment in a gym or in an athletic training centre. To observe mechanical waves in a jump rope, one end of the rope is tied to a rigid holder, while the other end is kept idle. The leisure end of the rope is handed over to the player. The player then moves the rope up and down, causing it to form crests and troughs, thereby displaying the action of a longitudinal mechanical wave.
6. Seismic Waves
Seismic waves are the waves produced due to the movement of the tectonic plates of the earth or because of an earthquake. Since the propagation of seismic waves requires a rigid medium, they are classified under the category of mechanical waves.