Vibrations & Waves Advanced Physics Presentation

Waves  A wave is a disturbance which causes a transfer of energy  Mechanical Wave – need a medium in order to travel  Electromagnetic Waves – do not need a medium  Waves can be transverse or longitudinal

Transverse Waves  When the particles of the disturbed medium move perpendicularly to the wave motion.  Example Waves in the stretched strings of musical instruments Waves in the stretched strings of musical instruments Waves upon the surfaces of liquids Waves upon the surfaces of liquids Radio and Light waves Radio and Light waves

Longitudinal Waves  When particles move along the direction of the wave rather than at right angles to it.  The medium vibrates parallel to the direction of the energy transfer  Example Sound Waves Sound Waves

Characteristics of Waves  Frequency  Period  Wavelength  Amplitude  Speed  Trough  Crest

Crests & Troughs  The high points of the wave are called crests  The low points of the waves are called troughs.  Amplitude is the distance from the mid- point to the crest (or trough) of the wave.

Wavelength  Wavelength is described as the distance from the top of one crest to the top of the next one.  It is also the distance between successive identical parts of the wave.

Frequency  How often a vibration occurs is called the frequency.  The unit of frequency is called the hertz (Hz)  A frequency of one cycle per second is 1 hertz, two cycles per second is 2 hertz, and so on.

Frequency and Period  If the frequency of a cycle is known then the period can be calculated.  Therefore if the period is known then the frequency can be calculated.  The formula is: Frequency = 1/period or Period = 1/frequency

Question  Using the previous formula and information answer the following question. What is the frequency in vibrations per second of a 100-Hz wave? What is the frequency in vibrations per second of a 100-Hz wave? Hint: 1 cycle per second = 1 Hz Hint: 1 cycle per second = 1 Hz

Answer  A 100-Hz wave vibrates 100 times per second.

Question  The Sears Tower in Chicago sways back and forth at a frequency of about 0.1 Hz. What is its period of vibration?  Hint: period = 1/frequency

Answer  The period is 1/frequency = 1vib/0.1hz 1vib/0.1vib/s = 10s 1vib/0.1vib/s = 10s Each vibration takes 10 seconds Each vibration takes 10 seconds

Wave Speed  The speed of a wave depends on the medium through which it travels.  Wave speed equals frequency times wavelength.  Formula v=גf Where v is wave speed; ג (lambda) is wavelength and f is frequency.

Question  If a water wave vibrates up and down two times each second and the distance between wave crests is 1.5m, what is the frequency of the wave? What is its wavelength? What is its speed? Hint: v=גf Hint: v=גf

Answer  The frequency of the wave is 2Hz; its wavelength is 1.5m; and its wave speed is wavelength X frequency  = 1.5m * 2Hz = 3m/s

Question  What is the wavelength of a 340-Hz sound wave when the speed of sound in air is 340m/s?

Answer  The wavelength of the 340-Hz sound wave must be 1 m. Then wave speed = 1m * 340 Hz = 340m/s. Then wave speed = 1m * 340 Hz = 340m/s.