$($a$)$

Q$12.$The graphs in the figure on the right show the displacement caused by a wave moving along a string at two instants, $($a$)t_1$ and $($b$)t_2.$ Let $v_{av}$ denote the average speed of a piece of string during the time interval between $t_1$ and $t_2.$ Compare the wave speed $c$ to $v_{av}$ of a piece of the string at the position $x=a.$

$($b$)$