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Traveling waves cont. and 20.2. Wave Speed. A disturbance is created by a source. Once created, the disturbance travels outward through the medium at the wave speed (v) A wave transfers energy, but it does not transfer any material or substance outward from the source. Speed= distance/time.
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Wave Speed • A disturbance is created by a source. • Once created, the disturbance travels outward through the medium at the wave speed (v) • A wave transfers energy, but it does not transfer any material or substance outward from the source. • Speed= distance/time
Wave along a string • The wave speed on a string stretched with tension Ts is : • Where µ is the string’s mass to length ratio • µ = m/L • This is also called the linear density. • SI unit is kg/m
A fat sting has a larger value of µ than a skinny string made of the same material. • A steel wire has a larger value of µ than a skinny string made of the same diameter. • Assuming that the string is uniform. • You can increase the wave speed either by increasing the string’s tension or by decreasing the strings linear density (make it skinnier)
Example 20.1 • In general the wave speed is a property of the medium. • Wave speed depends on the restoring forces within the medium but not all on the shape or size of the pulse. How the pulse was generated or how far it has traveled.
One-dimensional waves • Waves graphically • 20.3 shows the strings displacement change in Y at a particular instant of time. As a function of position x along the string. • Think of it as a snap shot of the wave at a certain time.
20.4 sequence of snapshot graphs as it moves. • Wave pulse moves forward distance ∆x = v∆t • Tells us where the wave is and how it varies with position. • But only at one instant of time. • Snapshot gives no information about how the wave changes with time.
History graph • Follow the dot marked on the string • It can produce a graph showing how the displacement of this dot changes with time. • Shown in 20.5 • This is a history graph and tells the history of that particular point in the medium. • Does it seem backwards? It is correct why?
The snapshot graph tells us how thing look throughout all of space, but only at one instant of time. • The history graph tells us how things look at all times, but only at one position in space. • Example 20.2 • Stop to think 20.2
