To propagate both longitudinal and transverse waves, a material must have

A train whistling at constant frequency $$n$$ is moving towards a station at a constant speed $$v$$. The train goes past a stationary observer on the station. The frequency $$n$$ of the sound as heard by the observer is plotted as a function of time $$t$$. Identify the correct curve.

The equation of a stationary wave is $$y=2 \sin \left(\frac{\pi x}{15}\right) \cos (48 \pi t)$$. The distance between a node and its next antinode is