Question

Two waves are interfering with each other. The first wave has an amplitude of +2 cm, and the second wave has an amplitude of -1 cm. When these two waves superpose, what is the amplitude of the superposition?

_____ cm
+1
+3
-3
no change

Answer 1

There's no such thing as a wave with negative amplitude.

If these two waves have the same frequency and meet in phase,
then the amplitude of the superposition is 3 cm.

If they have the same frequency and meet out of phase,
then the amplitude of the superposition is 1 cm.

If they have the same frequency and meet at some intermediate phase, then
the amplitude of the superposition is something between 1 cm and 3 cm.

If the two waves do not have the same frequency, then their superposition
is a complicated combination of both frequencies and both amplitudes.

Answer 2

The positive and negative signs of amplitude implies that the particle displaces in +y direction and -y direction respectively.
Therefore the amplitude of the resultant wave = 2-1 = 1 cm.