Two identical pulses are moving in opposite directions along a stretched string that has one fixed end and the other movable, as shown in (Figure 1). The two pulses reflect off the boundaries of the string, and at some later time, they pass through the middle of the string and interfere.

1. Consider the point where the two pulses start to overlap, point O in (Figure 2) . What is the displacement of point O as these pulses interfere?

It varies with time.
It remains zero.
It depends on the (identical) amplitude of the pulses.
It is zero only when the pulses begin to overlap.

2. Why does destructive interference occur when the two pulses overlap instead of constructive interference?

because the pulses are traveling in opposite directions
because a pulse is inverted upon reflection
because the pulses are identical and cancel each other out
because constructive interference occurs only when the pulses have the same amplitude

3. As the pulses interfere destructively there is a point in time when the string is perfectly straight. Which of the following statements is true at this moment?

The energy of the string is zero.
The string is not moving either up or down.
The string has only kinetic energy.
1. It remains zero

2. because a pulse is inverted upon reflection

3. The string has only kinetic energy