Q: Two wave pulses identical in shape but inverted with respect to each other are produced at the two ends of a stretched string and are moving in opposite directions with same speed. At an instant when the pulses reach the middle of the string, the string becomes completely straight. What happens to the energy of the two pulses at this instant?

A: When the two identical wave pulses reach the middle of the stretched string, they superimpose and interfere with each other. Since the two pulses are exactly the same, but inverted with respect to each other, they  cancel each other out completely, resulting in a straight string .

At this instant, the energy of the pulses is not lost but rather it is redistributed among the various degrees of freedom of the string’s molecules. The energy is transformed into kinetic and potential energy of the string’s molecules, causing them to vibrate and oscillate about their equilibrium positions. Therefore, even though the wave pulses are no longer present and the string appears to be motionless, the energy that was once stored in the wave pulses has been converted into other forms of energy and is still present in the system.