Q.
A second harmonic has to be generated in a string of length l stretched between two rigid supports. The points where the string has to be plucked and touched are respectively
In the 2nd harmonic the shape of wave looks like.
The point of touching forms a node point of plucking forms an antinode. Hence, for 2nd harmonic, we need to need wire touched at 2l and pluck it at 4l.