Physics · Ch 11 — Waves
Ripples and Wave Formation on the Water Surface
Ripples and Wave Formation on the Water Surface
Dropping a stone into a trough of still water produces a disturbance that spreads outward from the point of impact as a set of concentric, ever-widening circular ripples. This spreading happens because some of the kinetic energy carried by the falling stone is transferred to the water molecules right at the surface where it strikes, and that energy then propagates outward through the surface as a wave. Crucially, the water molecules that make up the medium do NOT themselves travel outward along with the ripple -- this can be verified directly by placing a small strip of paper on the water surface some distance from the impact point: as each ripple passes beneath the paper strip, the strip is simply seen to move up and down, oscillating about its original position, never drifting outward with the wave. This single observation is the clearest possible demonstration of what a wave actually is: the water surface as a whole executes vibratory m …
What this figure shows. Three everyday examples of waves are shown side by side: panel (a) shows ocean waves rolling in and breaking near a seashore, with their characteristic repeating crest-and-trough profile visible along the water's surface; panel (b) shows a rubber band stretched between two fingers and plucked, so that it vibrates back and forth forming a standing-wave-like pattern along its length; panel (c) shows the concentric circular ripples spreading outward across a water surface after a disturbance, exactly as produced by a stone dropped into a still trough. Together the three panels establish that "wave" is a broad category covering very different physical situations -- large-scale ocean swells, a taut vibrating band, and small ripples -- all sharing the same underlying feature of a travelling disturbance carr …
What this figure shows. A single stone is shown at the instant it strikes the still surface of a trough of water, with the resulting disturbance already spreading outward as a set of concentric circular ripples of ever-increasing radius centred on the point of impact. The figure is the chapter's opening visual argument for the defining property of a wave: the ripple pattern clearly propagates outward across the whole surface, yet a floating marker (such as a small paper strip) placed anywhere on the surface would only be observed to bob vertically up and down in place as each ripple passes beneath it, never drifting outward with the ripple itself -- demonstrating that it is the disturbance, and the kinetic …