Waves Part 1 Wave Good Bye to Mechanics

First things first A pulse is a single disturbance that moves through a medium Wave motion or Periodic motion is a series of regularly occurring waves or pulses Da Vinci said, “it often happens that the wave flees the place of creation, while the water does not; like the waves made in a field of grain by the wind, where we see the waves running across the field while the grain remains in place. Physics: waves transfer kinetic energy along the medium without motion of the medium as a whole

Pre Req’s Crest and trough of a wave –formally known as antinodes Points at equilibrium are called nodes

Types of waves Mechanical –water, sound, seismic –requires medium to transmit Electromagnetic –light, radio, x rays –travel at speed of light; does not require medium Matter –electrons, protons, fundamental particles –atoms and molecules travel as waves

Two forms Waves come in two forms –transverse displacement of particle is perpendicular to the direction of the wave water, string –longitudinal displacement of particle is parallel to the direction of the wave sound, slinky We concentrate on transverse waves

Old friends and new ones Frequency and period –# of cycles per unit time –time for one complete cycle –note: cycle = repetition Wavelength, –distance between repetitions of the wave form (shape) Amplitude –maximum displacement from equilibrium

Speed kills A traveling waves moves with a speed, v v= f –f is the frequency !Speed of waves is determined by the medium –however, it is related by the wavelength and frequency equation on board CP: use trig to describe waves

Crash into Me If two traveling waves move toward each other, they interfere If they are in phase, their amplitudes will constructively interfere If they are completely out of phase, their amplitudes will destructively interfere Two waves are said to be in phase if they have the same displacement at the same time

Waving Two examples:

I’m super, thanks for asking The principle of superposition states that the total displacement at any point where waves meet is equal to the algebraic sum of the displacements of each wave. –Special cases completely constructive completely destructive

Setting Boundaries If instead a single transverse wave encounters a open boundary, the reflected wave is unchanged If instead a single transverse wave encounters a fixed boundary, the reflected wave is inverted If a waves travels through a spring of different mediums (densities), part of the wave is transmitted; part of the wave is reflected

Waves are determined by their frequency –blue light has a different frequency than red light; low notes have different frequencies than high notes Every object has a characteristic (natural) frequency at which the object vibrates –this is called the resonant frequency

Does this resonate with you? Resonance –If an object is disturbed by a wave that matches the object’s resonant frequency, the amplitude of the wave increases. This causes the object to increase the amplitude of vibration. Tacoma Narrows Bridge

Things are not always as they seem The frequency you hear may not be the actual frequency –think of cars going by on the highway The apparent change in frequency caused by the relative motions of the source and the observer is known as the Doppler Effect. –Speed of sound in air at zero degrees Celsius = 330 m/sec –Speed of sound in air at room temp is 343 m/sec

Practically the end Practical uses of the Doppler Effect –sonar fishing especially –bats hunts prey by emitting a high frequency sound –Radar guns sort of