Rob Cartwright of Alexandra asks :-
When a marble is rolled over a flat surface why does it make a noise?
Alan Walton, of Cambridge University's Cavendish Laboratory and who specialises in how sound is generated in burbling brooks, responded.
Anyone who has ever walked though a forest floor in the autumn will know that they generate a lot of noise as their feet fracture fallen leaves and twigs. Sound is almost invariably emitted when brittle materials crack or break. You will certainly have heard the tinkle of breaking glass! This sound emission is actually used by engineers to detect the growth of cracks in engineering structures such as bridges; microphones attached to the structure can pick up the telltale sign of growing cracks. Virtually no sound is produced during the fracture of highly ductile materials like plastic bags or chewing gum.
When one object slides or rolls over a surface, friction comes into play. The frictional force arises - at least in part – as a result of the two surfaces ‘welding’ together when they are placed in contact. These ‘welds’ are very localised; at an atomic level even a polished surface will look mountainous so it might be like placing the Himalays face down on top of the Alps, the welds being formed where the peaks meet. As the surfaces slide or roll over each other these welds will have to snap, emitting sound in the process. The sound originates as the newly-formed surfaces oscillate in the way a loudspeaker cone oscillates. In brittle materials there is little internal damping of the oscillations in the material (unlike ductile materials). So some of the work we do as we slide or roll an object gets transformed into mechanical energy as we stretch and break these welds, which in turn gets transformed into sound energy and heat (this heat may indeed be sufficient to cause the surfaces to melt locally; an important part of the polishing process).
But another factor may be at work. We all know that a stuck drum skin emits sound when stuck. A ball rolling over a flexible surface will locally depress the surface before moving on. This is of course equivalent to striking the surface with a stick. This process of setting up oscillations in the surface may be the most significant one in some circumstances, such as when a marble rolls over a thin pane of glass supported only at its edges. If the glass rests on a rigid flat surface the bulk of the sound probably originates from ‘snapping welds’.
If you place a microphone in contact with the stationary surface you will be able to hear sounds that will remind you of the forest walk as the marble rolls across it.