Function doppler_effect_kernel 

pub fn doppler_effect_kernel(
    freq_source: &Frequency,
    wave_speed: &Speed,
    obs_speed: &Speed,
    src_speed: &Speed,
) -> Result<Frequency, PhysicsError>

Calculates the observed frequency due to the Doppler effect for longitudinal motion.

This kernel assumes the “Approaching” scenario where source and observer move towards each other. For receding motion, one might intuitively negate the speeds, but since Speed is non-negative, a separate function or directional flag would be safer. This implementation strictly valid for the Approaching case:

• Observer moving towards source (+vo)
• Source moving towards observer (-vs in denominator)

Formula: $f_{obs} = f_{src} \frac{v + v_o}{v - v_s}$

Arguments

• freq_source - Frequency emitted by the source.
• wave_speed - Speed of the wave in the medium ($v$).
• obs_speed - Speed of the observer relative to the medium ($v_o$).
• src_speed - Speed of the source relative to the medium ($v_s$).

Errors

• PhysicsError::MetricSingularity - If $v_s \ge v$, causing a sonic boom (denominator zero or negative).