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Waves, an Introduction

When you throw a rock into an otherwise calm lake, the circular ripples carry energy and momentum away from the original disturbance. These ripples are waves (Figure 11-9). When you see a ripple across a field of grain, that is not a wave, because it is the wind pushing the grain, and the grain itself does not carry any energy or momentum.

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Figure 11-9

A wave is a disturbance (small movement or change) in a medium such that, although the medium moves hardly at all. the disturbance travels a long distance, transporting energy.

Figure 11-10 shows water waves frozen at several moments in time. The waves are moving to the right. A water skater sitting at point P goes up and down and up with period T, measured in [s]. Of course, his frequency is f = 1/T. The wavelength λ is the length from peak to peak (or trough to trough or ascending zero point to ascending zero point). The wavelength is measured in [m]. The amplitude A is a measure of the size of the disturbance, measured from the equilibrium point to the high point. The units depend on the kind of wave it is and on how we measure it.

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Figure 11-10


The velocity of a wave is given by


In many cases the velocity is constant, and this formula simply relates frequency and wavelength. Watch for this on the MCAT.

In a wave, if the disturbance of the medium is in the same direction as the direction of wave travel, then the wave is called longitudinal. Figure 11-11 shows a longitudinal wave on a spring. The small arrows show the direction of the displacement, and the large arrow shows the direction of travel.

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Figure 11-11

If the disturbance of the medium is perpendicular to the direction of wave travel, then the wave is called transverse. Figure 11-12 shows two examples of transverse waves. Transverse waves are capable of being polarized, that is, they can be confined to moving in one of two dimensions. Unpolarized transverse waves are a random mixture of the two polarizations. Longitudinal waves cannot be polarized.

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Figure 11-12

The table below shows most of the examples of waves that you need to know for the MCAT.
wave longitudinal/transverse medium
water wave both water
wave on plucked string T string
sound L     air
earthquake both earth
light  T electric, magnetic fields

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