Waves

1. Waves The Nature of Waves

2. Waves Waves and Energy • A wave is a disturbance that transfers energy from place to place. • In science, energy is defined as the ability to do work. • To understand waves, think of a boat out on the ocean. • If a wave disturbs the surface of the water, it will cause anything floating on the water to be disturbed, too. • The energy carried by a wave can lift even a large ship as it passes.

3. Waves Waves and Energy • Many waves require something to travel through. • Water waves travel along the surface of the water, and sound waves travel through air. • You can even make a wave travel along a rope.

4. Waves Waves and Energy • The material through which a wave travels is called a medium. • Gases (such as air), liquids (such as water), and solids (such as ropes) all act as mediums. • Waves that require a medium through which to travel are called mechanical waves.

5. Waves Waves and Energy • Although waves travel through a medium, they do not carry the medium itself with them. • Look at the duck in the illustrations at the side. • When a wave moves under the duck, the duck moves up and down. • It does not move along the surface of the water. • After the wave passes, the water and the duck return to where they started.

6. Waves Waves and Energy

7. Waves Waves and Energy • Breaking waves at a beach behave a little differently. • When waves hit a beach, the water does move along with the wave. • This happens because the ocean floor near the beach slopes upward. • As the water at the bottom of the wave hits the slope, it moves up toward the top of the wave. • The top of the wave gets bigger and continues to move forward. • Eventually it topples over, turning white and frothy.

8. Waves What Causes Waves? • Waves are created when a source of energy causes a medium to vibrate. • A vibration is a repeated back-and-forth or up-and-down motion. • This motion is the wave

9. Waves What Causes Waves? • A moving object has energy. • The moving object can transfer energy • to a nearby medium, creating a wave. • For example, as the propellers of a motorboat turn, they disturb the calm water surface. • The boat’s propeller transfers energy to the water. • The propeller produces a wave that travels through the water. • As the boat moves through the water, it • also causes waves.

10. Waves Types of Waves • Different types of waves travel through mediums in different ways. • Waves are classified according to how they move. • The three types of waves are • transverse waves, longitudinal • waves, and surface waves.

11. Waves Transverse Waves • When you make a wave on a rope, the wave moves from one end of the rope to the other. • The rope itself, however, moves up and down or from side to side. • Waves that move the medium at right angles to the direction in which the waves are traveling are called transverse waves.

12. Waves Transverse Waves • Transverse means “across.” • As a transverse wave moves in one direction, the particles of the medium move across the direction of the wave. • The diagram below shows • that some parts of the • rope are very high while • some are very low. • The highest parts of the • wave are called crests, • and the lowest parts are • called troughs (trawfs).

13. Waves Longitudinal Waves • The photo below shows a different kind of wave. • If you stretch out a spring toy and push and pull one end, you can produce a longitudinal wave. • Longitudinal waves (lawn juh too duh nul) move the particles of the medium parallel to the direction in which the waves are traveling. • The coils in the spring • move back and forth • parallel to the wave • motion.

14. Waves Longitudinal Waves • Notice in that in some parts of the spring the coils are close together. • In other parts, the coils are more spread out. • The parts where the • coils are close • together are called • compressions • (kum presh uns). • The parts where the • coils are spread out, or • rarefied, are called • Rarefactions • (rair uh fak shunz)

15. Waves Longitudinal Waves • As compressions and rarefactions travel along the spring toy, each coil moves slightly forward and then back. • The energy travels from one end of the spring to the other, creating a wave. • After the wave passes, each part of the spring returns to the position where it started.

16. Waves Surface Waves • Surface wavesare combinations of transverse and longitudinal waves. • These waves occur at the surface between two mediums, such as water and air. • When a wave passes through water, the water (and anything on it) moves up and down, like a transverse wave on a rope. • The water also moves back and forth slightly in the direction that the wave is traveling, like the coils of the spring.

17. Waves Surface Waves • But unlike the coils of a spring, water does not compress. • The up-and-down and back-and-forth movements combine to make each particle of water move in a circle. • The diagram below shows the circular motion of surface waves.

18. Waves Properties of Waves • There are many different kinds of waves. • Waves can carry a little energy or a lot. • They can be short or long. • They can be rare or frequent. • They can travel fast or slow. • All waves, however, share certain properties. • The basic properties of waves are amplitude, wavelength, frequency, and speed

19. Waves Properties of Waves • To understand the properties of waves, it helps to represent a wave on a diagram. • You can draw a transverse wave as shown in the diagram below. • Think of the dashed line as the position of the rope before it is disturbed. This is its rest position. As the wave passes, the rope • goes above or below • the rest position. • Remember that the crests • and the troughs are the • highest and lowest points • on the wave.

20. Waves Properties of Waves • To draw longitudinal waves, think of the compressions in the spring toy as being similar to the crests of a transverse wave. • The rarefactions in the spring toy are like the troughs of a transverse wave. • By treating • compressions as • crests and • rarefactions as • troughs, you can • draw longitudinal • waves in the same way as transverse waves.

21. Waves Properties of Waves • Amplitude is the maximum distance the particles of the medium carrying the wave move away from their rest positions. • The amplitude is a measure of how much a particle in the medium moves when disturbed by the wave. • The amplitude of a • water wave is the • maximum distance a • water particle moves • above or below the • surface level of calm • water.

22. Waves Properties of Waves • Waves are produced by something vibrating. • The farther something moves as it vibrates, the larger the amplitude of the resulting waves. • You can increase the amplitude of the waves on a rope by moving your hand up and down a greater distance. • This greater amount of • energy is then transferred • to the rope. Thus, the • amplitude of a wave is a • direct measure of its • energy.

23. Waves Properties of Waves • Compare the two transverse waves in the diagram below. You can see that wave A goes up and down a greater distance than wave B. • The amplitude of a transverse wave is the maximum distance the medium moves up or down from its rest position. • You can find the amplitude of a transverse wave by measuring the distance from the rest position to a crest or to a trough.

24. Waves Properties of Waves • The amplitude of a longitudinal wave is a measure of how compressed or rarefied the medium becomes. • High-energy vibrations cause the compressions to be very crowded. • This makes the rarefactions quite loose. • Crowded compressions and uncrowded rarefactions are like high crests and low troughs. • They mean that the longitudinal wave has a large amplitude.

25. Waves Properties of Waves

26. Waves Properties of Waves • A wave travels a certain distance before it starts to repeat. • The distance between two corresponding parts of a wave is its wavelength. • You can find the wavelength of a transverse wave by measuring the distance from crest to crest or from trough to trough. • You can find the wave- • length of a longitudinal • wave by measuring the • distance from one • compression to the • next.

27. Waves Properties of Waves • Wave frequency is the number of complete waves that pass a given point in a certain amount of time. • If you make waves on a rope so that one wave passes by every second, the frequency is 1 wave per second. • Since waves are vibrations of a medium, frequency can also be described as the number of vibrations per second.

28. Waves Properties of Waves • Frequency is measured in units called hertz. • A wave or vibration that occurs every second has a frequency of 1 Hz. • If two waves pass you every second, then the frequency of the wave is 2 per second, or 2 hertz. • The hertz was named after the German scientist Heinrich Hertz, who first produced radio waves in an experimental situation.

29. Waves Properties of Waves

30. Waves Properties of Waves The speed, wavelength, and frequency of a wave are related to each other by a mathematical formula. If you know any two of the quantities in the speed formula—speed, wavelength, and frequency—you can calculate the third quantity.

31. Waves Properties of Waves • For example, if you know the speed and the wavelength of a wave, you can calculate the frequency. • If you know the speed and the frequency, you can calculate the wavelength.

32. Waves Properties of Waves • Waves in different mediums travel at different speeds. • In a given medium and under the same conditions, the speed of a wave is constant. • For example, all sound waves traveling through the air at the same pressure and at the same temperature travel at the same speed. • If the temperature or pressure changes, the sound waves will travel at a different speed.

33. Waves Properties of Waves