Displacement against time

A wave is a disturbance moving from place to place, so both time and distance are part of it. Start with time: follow a single particle as the wave passes.

Real waves usually vibrate in complicated ways. This lesson looks only at the simplest shape of wave, the sine wave.

After Figure 4.10 — the displacement of one particle from its rest position, against time. Here the period is 4 s and the amplitude 2 cm.

Reading the graph

1. At t = 0 the particle is at its rest position: displacement zero.
2. It moves up until, at t = 1 s, it reaches its greatest positive displacement.
3. It comes back to zero at t = 2 s and carries on the other way.
4. At t = 3 s it reaches its greatest negative displacement.
5. At t = 4 s it is back at zero, and the motion repeats.