Which of the following is a type of wave that is both mechanical and longitudinal?

1. Analyze the question

The question asks us to identify a wave that possesses two specific characteristics:

• Mechanical: It requires a material medium (solid, liquid, or gas) to propagate.
• Longitudinal: The particles of the medium vibrate parallel to the direction of wave propagation.

2. Explain the concept

• Mechanical waves: These waves need a medium to travel. Without particles to vibrate (like in a vacuum), they cannot exist. Examples include sound and water waves.
• Electromagnetic waves: These do not need a medium and can travel through a vacuum. Examples include light, X-rays, and radio waves.
• Longitudinal waves: In these waves, the particles move back and forth in the same direction the energy is traveling (creating compressions and rarefactions).
• Transverse waves: In these waves, the particles move perpendicular (at a 90-degree angle) to the direction of the wave.

3. Show the solution

Let’s evaluate the options:

• A. Water waves: These are a combination of longitudinal and transverse motion (surface waves). They are mechanical, but not purely longitudinal.
• B. Seismic waves: These include both P-waves (longitudinal) and S-waves (transverse). Because they are a mix, they are not the best fit for a general classification.
• C. Sound waves: Sound is produced by vibrations that create regions of high pressure (compressions) and low pressure (rarefactions) in a medium. Because the air molecules move back and forth in the same direction the sound is traveling, it is longitudinal. Since sound cannot travel through a vacuum, it is mechanical.
• D. Light waves: Light is an electromagnetic wave. It is transverse and does not require a medium.

4. Identify the answer

Sound waves perfectly fit the criteria of being both mechanical (requiring a medium) and longitudinal (vibrating parallel to the direction of travel).

5. Exam Tip

Remember the acronym "SLM" for Sound = Longitudinal = Mechanical. Conversely, remember that all electromagnetic waves (like light) are transverse and do not need a medium.