Chapter 17 Mechanical Waves

Transcription

1 Pearson Prentice Hall Physical Science: Concepts in Action Chapter 17 Mechanical Waves 17.1 Mechanical Waves Objectives: 1. Explain what causes mechanical waves 2. Name and describe the three main types of mechanical waves Bellringer What is your definition of a wave? What s in a wave? A wave is a repeating disturbance or movement that transfers energy through matter or space. Examples? Waves and Energy 11.1 Waves and Matter Because it is moving, the falling pebble has energy. As it splashes into the pool, the pebble transfers some of its energy to nearby water molecules, causing them to move. What you see is energy traveling in the form of a wave on the surface of the water. Imagine you re in a boat on a lake. Approaching waves bump against your boat, but they don t carry it along with them as they pass. 1

2 Waves and Matter The waves don t even carry the water along with them. Only the energy carried by the waves moves forward. A wave will travel only as long as it has energy to carry. All waves have this property they carry energy without transporting matter from place to place. Making Waves 11. Making Waves It is the up-and-down motion of your hand that creates the wave. Anything that moves up and down or back and forth in a rhythmic way is The vibrating. movement of your hand at the end of the rope created the wave. In fact, -All waves are produced by something that vibrates. What Causes Mechanical Waves Def: A wave is a disturbance that carries energy through space or matter Def: What a wave travels through is the medium (sound-air, earthquake-ground) Def: Waves that require a medium to travel are called mechanical waves (majority of waves) Mechanical waves carry energy from one place to another by using matter (a medium) Mechanical waves are started with a vibration Mechanical Waves Medium- matter the waves travel through. The medium can be a solid, a liquid, or gas 2

3 Types of Mechanical Waves The three main types of mechanical waves are transverse waves, (compressional) longitudinal waves and surface waves 11.1 Mechanical Waves mechanical waves- Waves that can travel only through a medium. Not all waves need a medium. Light and radio waves, can travel through space. (No Medium) Particles in a medium can vibrate up and down or back and forth as a wave moves by Def: If the particles move up and down they will move perpendicular to the direction of the wave-called a transverse wave Transverse waves take the shape of sine curves (looks like an s on its side) Def: High points are crests Def: Low points are troughs Def: Difference between high and low is called amplitude Bigger amplitudes mean more energy Def: in a longitudinal wave the medium moves in the same direction as the wave Transverse Waves transverse wave, Waves move at right angles to the direction that the wave travels. (Like water waves) For example, a water wave travels horizontally as the water moves vertically up and down. Bellringer What is a transverse wave? Compressional Waves compressional wave, waves move the same direction that the wave travels. You can model compressional waves with a coiled spring toy. 3

4 Compressional Waves Squeeze several coils together at one end of the spring. Then let go of the coils. Compressional Waves As the wave moves, it looks as if the whole spring is moving toward one The end. wave carries energy, but not matter, forward along the spring. Compressional waves also are called longitudinal waves. Def: In longitudinal waves the bunched area is a compression (think of a slinky) Def: The spaced out areas are called rarefactions Def: Amplitude on a longitudinal wave is maximum deviation from normal density or pressure At the boundary between two mediums (on the ocean for example) surface waves develop Def: Surface waves are combinations of both types of waves The particles in the medium of a surface wave move back and forth and up and down resulting in a circle, but they end up where they started Sound Waves Sound waves are compressional waves. When a noise is made, such as when a locker door slams shut and vibrates, nearby air molecules are pushed together by the vibrations. Sound Waves The air molecules are squeezed together like the coils in a coiled spring toy are when you make a compressional wave with it. The compressions travel through the air to make a wave. Sound in Other Materials Sound waves also can travel through other mediums, such as water and wood. When a sound wave reaches your ear, it causes your eardrum to vibrate. Your inner ear then sends signals to your brain, and your brain interprets the signals as sound. 4

5 Water Waves Ocean waves are formed most often by wind blowing across the ocean surface. The size of the waves that are formed depend on the wind speed, the distance over which the wind blows, and how long the wind blows. Seismic Waves Forces in Earth s crust can cause regions of the crust to shift, bend, or even break. The breaking crust vibrates, creating seismic (SIZE mihk) waves that carry energy outward. Seismic Waves Seismic waves are a combination of compressional and transverse waves. They can travel through Earth and along Earth s surface. The more the crust moves during an earthquake, the more energy is released. Click image to view movie Evidence of a Surface Transverse Wave Question 1 What is a wave? A wave is a repeating movement that transfers energy through matter or space. Question 2 Which is carried by a water wave? A. a boat on the surface B. boat anchor submerged 50 m C. energy D. water molecules 5

6 The answer is C. Waves carry energy without transporting matter from place to place. Question 3 Which type of wave does not need a medium? A. electromagnetic B. mechanical C. ocean D. sound The answer is A. Electromagnetic waves are made by vibrating electric charges and can travel through space where matter is not present. Bellringer What is the difference between a transverse and a compressional/longitudinal wave? 17.2 Properties of Mechanical Waves Objectives: 1. Explain what determines the frequency of a wave 2. Solve problems for frequency, wavelength and speed 3. Describe how amplitude and energy are related The Parts of a Wave Waves can differ in how much energy they carry and in how fast they travel. Waves also have other characteristics that make them different from each other. 6

7 The Parts of a Wave crests - Alternating high points troughs - Alternating low points The Parts of a Wave Parts of a compressional wave Compressionregion where the coils are close together. The Parts of a Wave Rarefaction- region where coils are spread apart Wavelength wavelength is the distance between two of the same points on a wave Wavelength 11.2 Wavelength The wavelengths of sound waves that you can hear range from a few centimeters for the highest-pitched sounds to about 15 m for the deepest sounds. 7

8 Frequency and Period frequency of a wave is the number of wavelengths that pass a fixed point each second. transverse waves- number of crests that pass by a point each second. Frequency is expressed in hertz (Hz). Frequency and Period Period- amount of time it takes one wavelength to pass a point. Wavelength is Related to Frequency As frequency increases, wavelength decreases. If you move the rope up, down, and back up in 1 s, the frequency of the wave you generate is 1 Hz. Sound and Mediums Sound travel faster in liquids and solids than they do in gases. Light waves travel more slowly in liquid and solids than they do in gases or in empty space. Sound waves faster in a material if the temperature of the material is increased. Frequency Def: periodic motion is any motion that repeats at regular time intervals Def: Frequency is how many waves pass by in a given time Frequency = 1/period = 1 / T = f It is measure in hertz (Hz) which is 1/s where s is seconds Humans can hear 20Hz to 20000Hz A wave s frequency equals the frequency of the vibrating source producing the wave Problems involving f, λ (wavelength) & speed Def: Wavelengths of waves are measured from one crest to the next (crest to crest OR trough to trough) or from one compression to the next It is represented by the Greek letter lambda whose symbol is λ The period of a wave is how long it takes for a complete wave to go by a spot, symbol is T Increasing the frequency (f) of a wave decreases the wavelength (λ) 8

9 Recall that speed = distance / time For waves we can use wavelength for distance (in meters, m) and period for time (s for seconds) Speed = wavelength X frequency v = λ * f The speed of a wave depends on what it travels through (medium) If the medium particles are closer together the energy from the wave can make vibrations easier So waves travels best in solids, then liquids, and worse in air Calculating Wave Speed You can calculate the speed of a wave represented by v by multiplying its frequency times its wavelength. Amplitude and Energy Def: amplitude is the difference between crest and the rest position or point of origin Def: the rest position or point of origin is an imaginary line through the middle of the wave that separates the crest from the trough The more energy a wave has, the greater its amplitude Amplitude and Energy Amplitude energy carried by a wave. The greater the wave s amplitude is, the more energy the wave carries. Click image to play movie Amplitude of Compressional Waves The closer the coils are in a compressi on, the farther apart they are in a rarefaction. Amplitude of Transverse Waves Draw Picture 9

10 Question 1 If a wave has a high point and a low point, is it a compressional or transverse wave? Transverse waves have alternating high points, called crests, and low points, called troughs. Question 2 What is the wavelength of a wave? A wavelength is the distance between one point on a wave and the nearest point just like it. Question 3 Which of the following refers to the number of wavelengths that pass a fixed point each second? A. frequency B. period C. wavelength D. wave speed The answer is A - Frequency. Period is a length of time, and wavelength is a distance. 10

11 17.3 Behavior of Waves Objectives: 1. Explain reflection and refraction and how they affect waves 2. Identify several factors that affect the amount of wave diffraction 3. Discuss two types of interference 4. Explain what a standing wave is and detail the wavelengths that produce it To Be a Wave All waves must- Reflect, Refract, and Diffract. Reflection and Refraction Def: Reflection occurs when a wave meets a boundary and bounces off The wave cannot pass through the surface Reflection does not change the speed or frequency of a wave, but the wave can be flipped upside down Def: Refraction is the bending of a wave as it travels through different mediums When a wave enters a medium at an angle, refraction occurs because one side of the wave moves more slowly than the other side (ex: pencil in water) Reflection How does the reflection of light allow you to see yourself in the mirror? It happens in two steps. First, light strikes your face and bounces off. Then, the light reflected off your face strikes the mirror and is reflected into your eyes. What about Sound Waves Echoes Echo - sound waves hit an object, they reflect and come back to you. You hear the sound multiple times. Draw this Picture: The Law of Reflection The beam striking the mirror is called the incident beam. The beam that bounces off the mirror is called the reflected beam. 11

12 The Law of Reflection The line drawn perpendicul ar to the surface of the mirror is called the normal. The Law of Reflection The angle formed by the incident beam and the normal is the angle of incidence. The angle formed by the reflected beam and the normal is the angle of refection. The Law of Reflection law of reflection- The angle of incidence is equal to the angle of reflection. All reflected waves obey this law. Refraction When a wave passes from one medium to another. It changes speed and direction. (Example: from air to water) If the wave is traveling at an angle when it passes from one medium to another, it changes direction, or bends, as it changes speed. Refraction Refraction- Bending of a wave caused by a change in its speed as it moves from one medium to another. Refraction of Light in Water Light waves travel slower in water than in air. This causes light waves to change direction and bend toward the normal When light waves travel from air to water, they slow down and bend toward the normal. 12

13 Refraction of Light in Water You may have noticed that objects that are underwater seem closer to the surface than they really are. In the figure, the light waves reflected from the swimmer s foot are refracted away from the normal and enter your eyes. Refraction of Light in Water However, your brain assumes that all light waves have traveled in a straight line. The light waves that enter your eyes seem to have come from a foot that was higher in the water. Diffraction and Interference Def: Diffraction is the bending of a wave around an obstacle A wave diffracts more if its wavelength is large compared to the size of an opening or obstacle The two types of interference are constructive interference and destructive interference Diffraction When waves strike an object, several things can happen. The waves can bounce off, or be reflected. If the object is transparent, light waves can be refracted as they pass through it. Waves also can behave another way when they strike an object. The waves can bend around the object. Diffraction Diffraction - an object causes a wave to change direction and bend around it. Diffraction and refraction both cause waves to bend. The difference is that refraction occurs when waves pass through an object, while diffraction occurs when waves pass around an object. Diffraction After they pass through the opening, the waves spread out. 13

14 Diffraction and Wavelength Examples of diffraction? Hearing Around Corners Light waves have a much shorter wavelength. They are hardly diffracted at all by the door. Diffraction of Radio Waves AM radio waves have longer wavelengths than FM radio waves do. Because of their longer wavelengths, AM radio waves diffract around obstacles like buildings and mountains. As a result, AM radio reception is often better than FM reception around tall buildings and natural barriers such as hills. Interference interference When two or more waves overlap and combine to form a new wave Constructive Interference constructive interference- waves add together. Constructive Interference 14

15 Destructive Interference destructive interference- waves cancel each other. The combo of two or more waves at the same place at the same time causes interference Def: When two crests meet the interference is constructive-the wave becomes more energetic-the amplitude increases Def: When a crest and a trough meet they cancel each other and decrease the amplitude destructive Standing Waves Interference may cause standing waves- Def: Standing waves appear not to move along the medium Has areas of destructive interference where there is no vibration called nodes Def: a node is a point on a standing wave that has no displacement from the rest position Areas of maximum interference called antinodes Def: an antinode is a point where a crest & trough meet midway between 2 nodes A standing wave forms only if half a wavelength or a multiple of half a wavelength fits exactly into the length of a vibrating cord Standing Waves standing wave - waves equal in wavelength and amplitude, travel in opposite directions, and interfere with each other. Standing Waves in Music When the string of a violin is played with a bow, it vibrates and creates standing waves. Some instruments, like flutes, create standing waves in a column of air. 15

16 Resonance resonance - object is made to vibrate by absorbing energy at its natural frequency If enough energy is absorbed, the object can vibrate so strongly that it breaks apart. Question 1 State the law of reflection. According to the law of reflection, the angle of incidence is equal to the angle of reflection. Question 2 is the bending of a wave caused by a change in its speed as it moves from one medium to another. The answer is C. The greater the change in speed is, the more the wave bends. A. diffraction B. diffusion C. refraction D. reflection Question 3 Which is the means by which you can hear around corners? A. diffraction B. diffusion C. reflection D. refraction The answer is A. Diffraction occurs when an object causes a wave to change direction and bend around it. 16