Question

state the 3 truths and the 1 lie
1. The resulting shape of the medium would be an upward displaced sine pulse with an amplitude of 2 units.

2. Constructive interference is observed at any location where the two interfering waves are displaced upward.

3. Destructive interference occurs when a sine pulse with a maximum displacement of +1 unit meets a sine pulse with a maximum displacement of -1 unit.

4. The two pulses destroy each other in a permanent condition when they are completely overlapped.

Answer 1

Truths: 1, 2, 3. Lie: 4.

Brief reasons:
- 1 is true: if two identical upward sine pulses (each amplitude +1) completely overlap, their displacements add and the resultant pulse has amplitude +2.
- 2 is true (in the usual linear-wave sense): constructive interference occurs where the instantaneous displacements of the two waves have the same sign (both upward or both downward), so they reinforce.
- 3 is true: if a +1 pulse meets a -1 pulse and they overlap completely, they cancel (destructive interference) at that instant.
- 4 is false: the cancellation is not permanent in a linear medium — it is only present while the pulses overlap; after passing through each other the original pulses reappear. (Only in nonlinear/absorptive situations could energy be permanently lost.)