Question

eli creates longitudinal waves in the spring toy.
select the true statements about longitudinal waves.
the matter that longitudinal wave travels through is called the medium.
if a longitudinal waves frequency decreases and its speed stays the same,
its wavelength decreases.
in a longitudinal wave, the wave and the particles of matter move in
parallel directions.
if a longitudinal waves frequency increases, the energy carried by the
wave increases.

Explanation:

• First statement: By definition, the matter a wave travels through is called the medium. This is true for longitudinal waves as well.
• Second statement: Using the wave formula \(v = f\lambda\) (where \(v\) is speed, \(f\) is frequency, and \(\lambda\) is wavelength), if \(v\) is constant and \(f\) decreases, \(\lambda\) must increase (\(\lambda=\frac{v}{f}\)), so this statement is false.
• Third statement: In a longitudinal wave, the particles of the medium move parallel to the direction of wave propagation.
• Fourth statement: The energy of a wave (for non - quantum mechanical cases, in the classical wave picture) is related to the amplitude. But for a wave, if we consider the quantum mechanical picture (e.g., for sound waves as a stream of phonons), the energy of a phonon is \(E = hf\) (where \(h\) is Planck's constant). In the context of wave energy in general (for waves like sound which are longitudinal), as frequency \(f\) increases, the energy carried by the wave (per unit volume etc., in a more general sense related to the wave's ability to do work) increases.

Answer:

The matter that a longitudinal wave travels through is called the medium; In a longitudinal wave, the wave and the particles of matter move in parallel directions; If a longitudinal wave's frequency increases, the energy carried by the wave increases.