An ambulance travels due east along a straight, level road at a constant speed of 20m⋅s−1. The siren on the ambulance emits a tone whose rest frequency is 700Hz. A cyclist rides 10m in front of the ambulance, moving in the same direction and at the same speed as the ambulance. Air is still with respect to the ground.
(a) Determine whether the cyclist hears a frequency that is greater than, less than, or equal to 700Hz. Provide a clear explanation based on the relative motion between the source and the observer.
(b) The ambulance driver increases the vehicle’s speed to 25m⋅s−1 while the cyclist maintains 20m⋅s−1. Qualitatively describe how the frequency heard by the cyclist changes as the ambulance accelerates. Justify your answer without performing a numerical calculation.