The equation of a stationary wave along a stretched string is given by $y=5 \sin \frac{\pi x}{3} \cos 40 \pi t$. Here $x$ and $y$ are in cm and $t$ in second. The separation between two adjacent nodes is

Longitudinal waves cannot

The displacement of a plane progressive wave in a medium, travelling towards positive x-axis with velocity 4 m/s at t = 0 is given by $$y = 3\sin 2\pi \left( { - {x \over 3}} \right)$$. Then the expression for the displacement at a later time t = 4 sec will be