Springing Into Action ~ Waves

Why Are We Talking About Waves? It was more fun to talk about the stars! Remember that wavelength is related to the frequency. And frequency is related to energy. And energy is correlated to temperature. So, if we know something about the waves emitted from a star, we can infer information such as the temperature, color, rate and direction of travel, age, etc!

Review: Amplitude = A Wave height = 2A Period = Τ Frequency = ƒ Wavelength = λ

How Fast Do Waves Move? Light Waves (electromagnetic waves) travel at about 300,000,000 m/s. This is true in a vacuum and is about the same in all gases. But what about other types of waves? Obviously water waves don’t move the same speed as light. Or even as fast as sound. So, wave speed is variable. But what makes it change? What factors affect the speed at which a wave travels through a medium? Do you have any guesses about this right now? If I give you a rope, a meter stick, and a stop watch, how could you measure the speed of a wave?

What Determines Wave Speed? A mini-lab Testable Question(s): What variable(s) affect the speed at which a wave travels through a medium? What variables could possibly affect the speed of a wave? Hypothesis: Wave speed will vary in the following ways: Varible 1: wave speed will ________ with increase in ___________ Variable 2: wave speed will ________ with increase in __________ Variable 3: wave speed will ________ with increase in __________

Which variable(s) affected the speed of your wave? Amplitude? Frequency? Wavelength? Medium? Only MEDIUM affects wave speed!!

Reflect & Connect 2. The speed of light through empty space is about 3x108m/s. The distance between the Earth and Sun is about 1.5x1011m (or how many AU?) If you were riding a wave of light from the Sun to the Earth, how long would it take you to get to Earth?   Set up the equation by looking at the units of m/s and m This becomes a division problem so that meters cancel out for seconds as the unit in the answer to the question: Cancel units of meters and solve 1.5X10¹¹ m = 0.5X 10³ = 500 sec 3X10 m/s

Introduction to Wave Speed During a storm, you will always see lightning before you hear the thunder. Why? If the speed of sound is about 340 m/s, how much faster is the speed of light than the speed of sound? Speed of light Speed of sound   It is important to know the speed of a wave, because this is the speed at which the energy it carries is transferred from one place to another.

Much Ado about Waves p. 475 P&P #1: Make a complete wave on a slinky 2a. What direction does the piece of tape move in relation to the wave? The tape moved up and down , the wave moves left to right Where is the tape after the wave has passed? The tape returns to its equilibrium (starting) position.

Much Ado about Waves p. 475 This type of wave is called a transverse wave because the direction of motion of the medium is perpendicular to the direction of motion of the wave.

Much Ado about Waves p. 475 Longitudinal waves (compressional waves) need matter through which to travel. They need a medium to compress, otherwise they cannot travel. What direction does the piece of tape move in relation to the wave? The tape moves left and right and the wave moves left and right

Much Ado about Waves p. 475 Longitudinal waves (compressional waves) compress matter as they travel through a medium

Much Ado about Waves p. 475 What happens when the frequency of the wave is increased (more waves per second)? The wavelength gets smaller What happens when the frequency is low (fewer waves per second)? The wavelength is longer

Much Ado about Waves p. 475 What is the relationship between frequency and wavelength? Inverse relationship… when one goes up , the other goes down

Transverse & Longitudinal Waves Transverse Waves: http://upload.wikimedia.org/wikipedia/commons/7/74/Simple_harmonic_motion_animation.gif The disturbance is perpendicular to the direction the wave travels. Stringed instruments like guitars, violins, banjos Radio waves, light waves, microwaves  Longitudinal Waves aka Compression Waves (seismic “P” waves) The disturbance is parallel to the direction the wave travels. Sound waves (most of them)  Both Transverse and Longitudinal Waves GIF Neither Transverse or Longitudinal Water waves http://upload.wikimedia.org/wikipedia/commons/b/be/Shallow_water_wave.gif  

What is a ‘Medium’? A medium is a substance or material that carries the wave. You might have heard the phrase ‘news media’. This refers to the various institutions (newspaper offices, television stations, radio stations, etc.) that carry news from one location to another. The news moves through the media. The media doesn't make the news and the media isn't the ‘news’. The news media is merely the thing that carries the news from its source to various locations. In a similar manner, a wave medium is the substance that carries a wave (or disturbance) from one location to another. The wave medium is not the wave and it doesn't make the wave; it merely carries or transports the wave from its source to other locations.

A Wave Transports Energy When a wave is present in a medium (that is, when there is a disturbance moving through a medium), the individual particles of the medium are only temporarily displaced from their rest position. There is always a force acting upon the particles that restores them to their original position. In a slinky, each coil of the slinky ultimately returns to its original position. In a water wave, each molecule of the water ultimately returns to its original position. And in a stadium wave, each fan in the bleacher ultimately returns to its original position. The particles of the medium (water molecules, slinky coils, stadium fans) simply vibrate about a fixed position as the pattern of the disturbance moves from one location to another location. 

A Wave Transports Energy and Not Matter As a wave moves through a medium - from one particle to its adjacent particle - energy is being transported from one end of the medium to the other. In a slinky wave, a person imparts energy to the first coil by doing work upon it. The first coil receives a large amount of energy that it subsequently transfers to the second coil. When the first coil returns to its original position, it possesses the same amount of energy as it had before it was displaced. The first coil transferred its energy to the second coil. The second coil then has a large amount of energy that it subsequently transfers to the third coil. When the second coil returns to its original position, it possesses the same amount of energy as it had before it was displaced. This process of energy transfer continues as each coil interacts with its neighbor. In this manner, energy is transported from one end of the slinky to the other, from its source to another location.

Electromagnetic Spectrum

Check Understanding: Minute after minute, hour after hour, day after day, ocean waves continue to splash onto the shore. Explain why the beach is not completely submerged and why the middle of the ocean has not yet been depleted of its water supply. Ocean waves do not transport water. For the most part, an ocean wave does not bring any new water molecules from the middle of the ocean to shore. Ocean waves can only bring energy to the shore; the particles of the medium (water) simply oscillate about their fixed position. Therefore, water does not pile up on the beach.