Question

Two pulses are moving along a string. One pulse is moving to the right and the second is moving to the left. Both pulses reach point P at the same instant.

An illustration with a rectangular crest moving toward the right and a triangular trough going toward the left both toward point P and equidistant from P.
Which diagram shows the appearance of the string after the pulses pass point P?

An illustration with the triangular trough in the middle of the rectangular crest to the left of point P.
An illustration with the triangular trough in the middle of the rectangular crest to the right of point P.
An illustration with a rectangular crest moving toward the right past P and a triangular trough going toward the left past P.
An illustration with a rectangular crest moving toward the left and a triangular trough going toward the right both away from point P and equidistant from P.

Answer 1

The third diagram.

By linear superposition the pulses pass through each other: they briefly add while overlapping (possibly canceling at P if amplitudes are equal and opposite) but emerge unchanged and continue on. So after passing P the rectangular crest will be on the right moving right and the triangular trough on the left moving left.