Ch. 17 – Mechanical Waves & Sound

Section 17.1 & 17.2

17.1 - Mechanical Waves• Mechanical Wave: a disturbance in matter

that carries energy from one place to another.– Requires matter to travel through (medium)

• Medium = solids, liquids or gases• Outer space = empty (no medium)

– Created when a source of energy causes a vibration to travel through a medium

– 3 main types of mechanical waves:1.Transverse2.Longitudinal3.Surface

Transverse Waves

• Def. – a wave that causes the medium to vibrate at right angles to the direction in which the wave travels.

• Crest = highest point on a wave

• Trough = lowest point on a wave

Longitudinal Wave• Def.- a wave in which

the vibration of the medium is parallel to the direction the wave travels

• Compression = an area where the particles in a medium are spaced close together

• Rarefaction = an area where the particles in a medium are spread out

Surface Wave• Def. – a wave that travels along a

surface separating 2 media

• Combination of transverse and

longitudinal waves

• Example – ocean waves (between

water and air)

17.2 – Properties of Mechanical Waves• Frequency & Period

–Periodic motion – any motion that repeats at regular time intervals

–Period – the time required for one cycle; a complete motion that returns to its starting point (crest-to-crest)

–Frequency – the number of complete cycles in a given time

• Measured in cycles per second [hertz (Hz) = 1/sec]

• Wavelength (λ – lambda)– Def. = the distance between a point on one wave

and the same point on the next cycle of waves• Crest to crest on transverse wave• Compression to compression on longitudinal wave

– Increasing the frequency of a wave decreases its wavelength (See below)

Period

Wavelength Speed

(Hz)Frequency (m) Wavelength(m/s) Speed

• Wave Speed (v)

• Speed can change:- When entering a new medium- When pressure and temperature change

• If wave speed is constant then:− Wavelength is inversely proportional to

frequency (as wavelength increases, frequency decreases)

• Amplitude– Def. – the maximum displacement of the medium

from its rest position– Transverse waves

• The distance from the rest position to a crest or a trough

– Longitudinal waves• The maximum displacement of a point from its rest

position

– The more energy a wave has, the greater its amplitude (↑energy = ↑amplitude)