Chapter 25 Waves Conceptual Physics Chapter 25 1

Chapter 25 Waves Conceptual Physics Chapter 25 1

Vibration of a Pendulum ¤ The back-and-forth motion of a pendulum demonstrates a vibration. ¤ The time to complete one back-and-forth motion is called the period, T. The period is usually measured in seconds. ¤ The frequency, f, is a measure of how often the pendulum swings back and forth and is measured in cycles per second or Hertz (Hz). Conceptual Physics Chapter 25 2

Vibration of a Pendulum ¤ The period is not dependant on the mass of the pendulum nor is it dependant on the initial amplitude. ¤ The period is only dependant on the length of the pendulum and the acceleration of gravity. T = 2π L g Conceptual Physics Chapter 25 3

Question A pendulum completes five back-andforth cycles in 15 seconds. What is the frequency period of the pendulum? Conceptual Physics Chapter 25 4

Sine Wave ¤ The A sine high amplitude, wavelength, wave points can. A, are be λ, isis created called thevertical distance crests from the and simple the distance between lowharmonic points successive from are the motion called equilibrium, identical oftroughs. a mass parts or home on attached position, the wave. to tothe a spring. crest or trough. crests troughs Conceptual Physics Chapter 25 5

Energy Transfer by Waves ¤ Waves are a disturbance that travel through a medium. ¤ The source of all waves is something that vibrates. ¤ The wave carries energy away from the source. ¤ The disturbance moves along the medium; the medium itself does not move. ¤ The energy associated with the wave is dependant on the amplitude of the wave. Conceptual Physics Chapter 25 6

Classification of Waves ¤ All waves can be classified as either mechanical waves or electromagnetic waves. ¤ Mechanical waves rely on a material medium in which to propagate. E. g. , sound waves, water waves, slinky waves. ¤ Electromagnetic waves are able to transmit energy through empty space. E. g. , light waves, radio waves, microwaves, X-rays. ¤ Our focus in this chapter will be on mechanical waves. Conceptual Physics Chapter 25 7

Classification of Waves ¤ Another way to classify waves is on the basis of the direction of movement of the individual particles of the medium relative to the direction in which the waves travel. ¤ Three notable categories are transverse waves, longitudinal waves and surface waves. Conceptual Physics Chapter 25 8

Transverse Waves Thea wave In transverse propagates wave the fromparticle left to right displacement is perpendicular to the while the particles simply oscillate up direction wave propagation. and down of – the particles do not travel with the wave! direction of wave travel direction of particle motion Conceptual Physics Chapter 25 9

Longitudinal Waves Thea wave In longitudinal propagates wave from the particle left to right displacement is parallel to the direction while the particles oscillate back and of wave propagation. forth about their equilibrium positions direction of wave travel direction of particle motion Conceptual Physics Chapter 25 10

Slinky Waves A slinky can be used to produce either a longitudinal wave or a transverse wave. Conceptual Physics Chapter 25 11

Surface Waves As a water waveare propagates fromof left to Surface waves a combination right across the surface of the water, both longitudinal and transverse waves. the particles move in clockwise circles. The radius of the circles decreases at greater depths beneath the surface. Conceptual Physics Chapter 25 12

Wave Speed If we count the number of wave crests that pass the bird each vsecond = fλ (the frequency) and observe the distance between successive ¤ If Thethe speed frequency of the of is wave the increased, waves is dependant is the an inherent wavelength solely on crests (the wavelength) we can find the be property will medium reduced of through theproportionately waves which and is the dictated wave whiletravels. the by the wave horizontal distance the waves move each device remains speed or eventconstant! that generates the wave second (the wave speed). Conceptual Physics Chapter 25 13

Interference ¤ Unlike matter, multiple waves can occupy the same space at the same time. ¤ When waves interact, they generate interference patterns. ¤ Within the pattern, wave effects may be increased, decreased or neutralized. Conceptual Physics Chapter 25 14

Interference ¤ When Regardless, the waves the interference are out in phase of phase (crest pattern (crest matches is with crest), matches temporary. with The trough), wavesthe undergo passwaves through constructive undergo one interference destructive another withinterference no andpermanent reinforce and effect one partially another. to their or fully cancel one another. amplitude, wavelength, frequency or speed. Run Simulation Conceptual Physics Chapter 25 15

Interference ¤ Interference patterns can develop when water waves pass through one another. Notice the regions of alternating constructive and destructive interference. Conceptual Physics Chapter 25 16

Standing Waves ¤ A standing wave can be produced as a result of interference between two waves of equal amplitude and wavelength passing through one another. ¤ In a standing wave, the nodes result from destructive interference. These nodes remain stationary. ¤ The antinodes are points of maximum displacement which result from constructive interference. ¤ Nodes are always separated by one-half wavelength. Run Simulation Conceptual Physics Chapter 25 17

Standing Waves ¤ Different modes of vibration can be produced in the same medium under different conditions. The easiest standing wave to produce one Shaking the ropehas with segment. twice the frequency will produce the a standing wave Shaking rope with two segments. three times the frequency will produce a standing wave with three segments. Conceptual Physics Chapter 25 18

The Doppler Effect ¤ The Doppler effect is observed when a wave source is moving relative to the observer of the waves. ¤ There is an apparent upward shift in frequency for observers towards whom the source is approaching. ¤ There is an apparent downward shift in frequency for observers from whom the source is receding. ¤ The Doppler effect can be observed for any type of wave - water wave, sound wave, light wave, etc. Conceptual Physics Chapter 25 19

The Doppler Effect ¤ A bug bobbing up and down in the middle of a quiet pond produces circular waves that travel outward in all directions. The wave fronts form concentric circles of increasing radius. ¤ Waves encounter point A as frequently as they encounter point B. ¤ The frequency at both points is equal to the bobbing frequency of the bug. Conceptual Physics Chapter 25 20

The Doppler Effect ¤ If the bug begins moving to the right, the wave fronts remain circular, but the pattern is distorted – the circular waves are no longer concentric. ¤ An observer at point B would now encounter the waves more frequently and an observer at point A would encounter the waves less frequently. Conceptual Physics Chapter 25 21

The Doppler Effect The Doppler effect is evident when you hear the changing pitch of a fire truck’s siren as it passes you. When the fire truck approaches you, the waves encounter you more frequently and you hear a higher pitch. When the fire truck moves away from you, you hear a drop in pitch because the waves are encountering you less frequently. Conceptual Physics Chapter 25 22

The Doppler Effect Conceptual Physics Chapter 25 23

Bow Waves ¤ When The faster the speed the source of themoves, sourcethe in anarrower medium is as great will be the as. Vthe that speed is produced. of the waves it produces, the waves begin to pile up – a bow wave is formed. Conceptual Physics Chapter 25 24

Shock Waves ¤ When a supersonic aircraft exceeds the speed of sound, three-dimensional spherical waves pile up and generate a conical shock wave. ¤ When this shock wave reaches an observer on the ground, a sonic boom is heard. Conceptual Physics Chapter 25 25

Shock Waves A large cloud of condensation forms as this F – 18 Hornet breaks the sound barrier. Conceptual Physics Chapter 25 26