Superposition principle
Also called Principle of superposition
The superposition principle says that where two or more waves overlap, the displacement at each point is the sum of the displacements each wave would produce on its own, and the waves then continue on unchanged.
AP Physics 2 EK 14.6.A.3: when two or more wave pulses or waves overlap, the resulting displacement can be determined by adding the individual displacements. This is called superposition. Add signed displacements, not magnitudes: a cm crest meeting a cm trough gives zero at that instant.
The second half of the idea matters as much as the first. EK 14.6.A.2: when two or more wave pulses or waves interact with each other, they travel through each other and overlap rather than bouncing off each other. Two pulses that momentarily flatten the string to nothing emerge afterwards with their original shapes and speeds. Waves do not collide.
Everything else in Topic 14.6 is a consequence. Constructive and destructive interference are the two signs of the sum (EK 14.6.A.4). Beats are superposition of two slightly different frequencies. A standing wave is superposition of a wave with its own reflection.
Where the shortcut breaks. Displacements add; energies and intensities do not. Two equal crests coinciding give twice the displacement and four times the local energy, because energy goes as amplitude squared. Superposition is also a property of the linear waves this course handles, not a conservation law.